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Copy 1 



OFFICIAL PUBLICATION OF 

IOWA STATE COLLEGE OF AGRICULTURE 

AND MECHANIC ARTS 

Vol. XVII. September 25, 1918 No. 17 



THE THEORY OF 
UNDERDRAINAGE 



W*&>:$6 



i 



By 
W. J. SCHLICK 




BULLETIN 50 
ENGINEERING EXPERIMENT STATION 



Ames, Iowa 



Acceptance for mailing at special rate of postage provided for in Section 1108, 
Act of October 3, 1917. Authorized September 23, 1918. 



PURPOSE OF THE STATION 

THE purpose of the Engineering Experi- 
ment Station is to afford a service, 
through scientific investigations, evolution of 
new devices and methods, educational techni- 
cal information, and tests and analyses of ma- 
terials : 

For the manufacturing and other engineer- 
ing industries of Iowa; 

For the industries related to agriculture in 
the solution of their engineering problems; 

For all people of the State in the solution of 
the engineering problems of urban and rural 
life. 



OFFICIAL PUBLICATION OF 

IOWA STATE COLLEGE OF AGRICULTURE 

AND MECHANIC ARTS 

Vol. XVII. September 25, 1918 No. 17 



THE THEORY OF 
UNDERDRAINAGE 



By 
W. J. SCHLICK 




BULLETIN 50 
ENGINEERING EXPERIMENT STATION 



Ames, Iowa 



Acceptance for mailing at special rate of postage provided for in Section 1103, 
Act of October 3, 1917. Authorized September 23, 1918. 



^ 



OARD OF EDUCATION <Q ^ 

» 

Members 



Hon. D. D. Murphy, President Elkader 

Hon. Geo. T. Baker Davenport 

Hon. Chas. R. Brenton Dallas Center 

Hon. P. K. Holbrook Onawa 

Hon. Edw. P. Schoentgen Council Bluffs 

Hon. H. M. Eicher Washington 

Hon. Frank P. Jones Villisca 

Hon. Paul Stillman Jefferson 

Hon. W. C. Stuckslager Lisbon 

Finance Committee 

Hon. W. R. Boyd, President Cedar Rapids 

Hon. Thomas Lambert Sabula 

Hon. W. H. Gemmill, Secretary Des Moines 



ENGINEERING EXPERIMENT STATION 
Station Council 

(Appointed by the State Board of Education) 

Raymond A. Pearson, LL. D President 

*Anson Marston, C. E Professor 

Louis Bevier Spinney, B. M. E Professor 

Samuel Walker Beyer, B. S., Ph. D Professor 

Warren H. Meeker, M. E Professor 

Fred Alan Fish, M. E. in E. E Professor 

Allen Holmes Kimball, M. S Professor 

Thomas Harris MacDonald, B. C. E. . . Chief Engineer, Iowa Highway Commission 

Station Staff 

Raymond A. Pearson, LL. D President Ex Officio 

* Anson Marston, C. E Director and Civil Engineer 

Samuel Walker Beyer, B. S., Ph. D Director, Mining Engineer and Geologist 

Charles S. Nichols, C. E Assistant to Director, Sanitary Engineer 

Louis Bevier Spinney, B. M. E Illuminating Engineer and Physicist 

Warren H. Meeker, M. E Mechanical Engineer 

Fred Alan Fish, M. E. in E. E Electrical Engineer 

Allen Holmes Kimball, M. S Architectural Engineer 

William J. Schlick, C. E .Drainage Engineer 

T. R. Agg, C. E , Highway Engineer 

John Edwin Brindley, A. M., Ph. D Engineering Economist 

*Max Levine, S. B Bacteriologist 

Roy W. Crum, C. E Structural Engineer 

*Homer F. Staley, M. A Ceramic Engineer 

D. C. Faber, E. E Mechanical, Electrical, and Industrial Engineer 

* John S. Coye, S. B Chemist 

J. S. Dodds, C. E Assistant Highway Engineer 

H. V. Wright, B. S. in Chem. E. Chemist 

*B. Kamrass Assistant Engineer, Road Materials 

A. O. Smith Mechanician 

ft. Of 9f 

*Leave of absence during the war. ,. , n „- «*«#• 

MAN 25 |919 



TABLE OF CONTENTS 



I. INTRODUCTION AND HISTORICAL SKETCH. 

Page 
Art. 1. Importance of Underdrainage in Iowa 5 

2. Early History of Underdrainage 5 

3. Purpose of this Bulletin 7 

4. Acknowledgments 8 

II. SOILS. 

Art. 5. Soil Areas in Iowa g 

6. Mechanical Composition of Soils 13 

Viewpoints of the Drainage Engineer and the Agriculturist 13 

Texture and Structure 13 

7. Soil Texture 13 

Soil Separates 13 

Soil Separates in Common Soils 13 

Soil Separates in Crop Adaptations 15 

Size of Soil Grains .' 15 

Effective Diameter .!!!!! 16 

Uniformity Coefficient '.'.'.'. 16 

Number of Soil Grains '.'.'.'. 17 

Surface Area of Soil Grains 17 

8. Soil Structure . ' [' ' ' 17 

9. Porosity 18 

Porosity and Size of Soil Grains I!.'."!!!.'!!!!!!!!!!!! 18 

Porosity and Structure 18 

10. Porosity and Perviousness ...... j 19 

III. SOIL MOISTURE. 

Art. 11. Source of Soil Moisture 19 

12. Moisture Content of Soils 20 

Moisture Content and Crop Production 20 

13. Forms of Soil Moisture ' 21 

14. Gravitational Moisture .'. 21 

Gravitational Moisture Content of Soils. ......'.'. '91 

The Watertable 23 

15. Capillary Moisture ' 23 

Capillary Moisture Content 23 

Available Capillary Moisture ' 25 

Capillary Moisture and Drainage 95 

16. Hygroscopic Moisture 05 

Hygroscopic Moisture Content ' gr. 

17. Loss of Soil Moisture .....'.'!!."!!!!!.'!.'.'!!! 26 

IV. SOIL WATER MOVEMENTS. 

Art. 18. Soil-water Movement and Underdrainage 9R 

19. Forms of Soil-water Movement 9c 

20. Thermal Movements '. 97 

Evaporation 9 7 

21. Capillary Movement ......... 27 

Form of Movement and Factors Governing it.' ! " ' 27 

Relation of Capillary Movement to Underdrainage ' 28 

Rate and Extent of Capillarv Movement. .. "' '98 

00 „ Capillary Movement and Depth of Underdrains .' 29 

22. Gravitational Movement o« 

Form of Movement '.'.'.. f q 

Channels for Movement [ .[ on 

Effect of Temperature and Barometric Pressure 90 

Effect of Soil Texture ,; 

Effect of Structure .' .' ™ 

The Viscosity of the Moving "Water. . . 00 

Do Tile Drains "Draw?" ..!..!!!!! 33 



23. 



xt * How Water Moves from the Surface to the Drain 

Natural Underdrainage, Seeps and Springs 94 

Rate of Groundwater Movement or 

Relation of Groundwater Movement to Underd'rainage" " " %r 



or t. I — £ ^ "»*ci uia.iiia.se, oeejjs a,iiu opringS 94 

25. Rate of Groundwater Movement or 

Rela T E, i ° n of Groundwater Movement to Underdrainage! '. ' %d 

Effect and Spacing of Laterals ,2 

Effect and Depth of Laterals , 

The Time Factor ..-{.., gg 



V. RUNOFF. 

Page 

Art. 27. Definitions 40 

28. Factors Affecting- Runoff 40 

Rainfall 41 

Topography of the Watershed 42 

Character of Soil 42 

Evaporation and the Transpiration of Plants 43 

Climate and the Seasons 43 

29. Economic Rate of Runoff 43 

30. Runoff Data and Values 44 

VI. FLOW IN UNDERDRAINS. 

Art. 31. Cause of Flow and Factors Affecting Rate 46 

32. Formulas for Flow in Underdrains 47 

Kutter's Formula 47 

Use of Kutter's Formula 49 

Poncelet's Formula and Elliott's Modifications 50 

Use of Elliott's Formula 51 

33. Advantages of Each of the Two Formulas 52 

VII. RESULTS OF UNDERDRAINAGE. 

Art. 34. Financial Results 53 

35. Physical Results 54 

36. General Results 56 

LIST OF FIGURES. 

1. Principal Soil Areas of Iowa 9 

2. Crop Adaptation and Composition of Some Soils 15 

3. Ideal Arrangement of Spherical Soil Particles 18 

4. Proportional Amounts of the Three Forms of Soil Moisture 21 

5. Diagram Illustrating the Condition Obtaining After a Rain When the Soil 

Pores Are Filled With Air 24 

6. Diagram Illustrating the Movement of Water from the Surface to the Drain 34 

7. Diagram Illustrating Effect of Closer Spacing of Laterals 38 

8. Capacities of Drains as Given by Kutter's Formula with "n"=.015. .(facing) 48 

9. Diagram Showing the Effect of Drainage Upon the Height of Corn 56 

LIST OF TABLES. 

I. Soil Separates 13 

II. Physical Composition of Common Soils 14 

III. Physical Composition of Common Soils 15 

IV. Properties of Common Soils 17 

V. Moisture Capacity of Soils 20 

VI. Moisture Content of Soils 22 

VII. Gravitational Water Capacities of Soils Near Hanford, Cerro Gordo 

County, Iowa 22 

VIII. Available Moisture in Soils 25 

IX. Per Cents of Hygroscopic Moisture at 21" C, or Approximately 70° F. . . . 26 
X. Pounds of Water Per Day Per Square Foot of Soil Raised from Differ- 

erent Depths 28 

XL The Coefficient of Roughness in Average Iowa Drains 48 



The Theory of^Underdrainage 

I. INTRODUCTION. 

1. Importance of Underdrainage in Iowa. There has probably 
been no single factor, in the agricultural and commercial development 
of Iowa, of greater importance than drainage, and particularly under- 
drainage. It has been estimated on good authority that the lands in 
Iowa needing drainage include at least 7,100,000 1 acres, and that 
this work will cost $400,000,000'-, a sum equal to the cost of the 
Panama Canal. Practically all of this 7,100,000 acres will require un- 
derdrainage to put it in the condition for maximum crop production. 

It seems from the best available estimates that not much over 50 
per cent of the ultimate drainage work in Iowa has been completed. 
One Governor of Iowa, in his inaugural address, estimated that up to 
1911, 125,000 3 miles of underdrains had been constructed in Iowa, 
at a probable cost of $105,000,000. Up to the same year, the drainage 
work in one Iowa County, Winnebago, was estimated to have cost 
$1,700,000*. Late in 1918 it was stated that when work then in 
course of construction was completed, Kossuth County would have 
expended more than $7,000,000 for drainage improvements. 

While these figures are only estimates, they are as reliable as any 
which are available, and they serve to show the vast importance of 
drainage work, and particularly of underdrainage work, in this state. 
It is probable that during the next decade at least two hundred mil- 
lions of dollars will be required for drainage work in Iowa, and that 
the major portion of this will be expended for underdrainage systems. 

2. Early History of Underdrainage. One of the earliest records of 
underdrainage is found in the writings of the Roman, Columnela, who 
lived in the first century of the Christian Era. The Drainage Journal, 
Vol. XXIV, gives the following translation from that writer's 
works* : "We know of two kinds of ditches, those which are wide 
open and those which are hidden.. For hidden ditches one will dig 
trenches three feet deep, which shall be filled with pebbles or pure 
gravel, and then the whole will be filled with earth taken from the 
trench." That the Roman peoples did not, however, fully realize the 
need for drainage, or its benefits, is shown by the existence of the 
noted Pontine Marshes at the time of the fall of the Roman Empire. 

In the middle of the seventeenth century, Walter Blith of England, 
in writing on drainage, said that drains to be efficient must be laid 



Proceedings, Iowa State Drainage Association, 1912. 
Proceedings, Iowa State Drainage Association, 1912. 
Proceedings, Iowa State Drainage Association, 1911. 
Proceedings, Iowa State Drainage Association, 1910. 
*Geo. M. Thompson, Proceedings, Iowa State Drainage Association, 1911. 



three or four feet deep. In these writings he recommended the use 
of trenches filled with fagots or stones. A little later, about 1730, 
James Smith of Deanston, Scotland, and Joseph Parks, Engineer for 
the Royal Agricultural Society of England, made numerous experi- 
ments which gave drainage development an added impetus. The En- 
cyclopedia Brittanica in speaking of Smith and his work states that 
"L_he insisted on the necessity of providing each field which needed 
draining at all with a system of underground channels, .and so near 
together that the whole rain at any time on the surface should sink 
down and be carried off by the drains." Smith believed that the dis- 
tance between drains should be regulated by the retentiveness of the 
soil, but recommended parallel drains, 10 to 40 feet apart and 30 inches 
deep. In his earlier works, he recommended trenches filled with 12 
inches of stones which would pass a three inch ring. In his later work 
he advised the use of burned clay tile, or "field pottery," in the shape 
of a letter U inverted and generally used without the bottom plate, 
or "sole," which was commonly used later. At about the same time 
Parks advocated drains further apart and at a depth of at least four 
feet. Both of these men were strong advocates of underdrainage, but 
they encountered and held different opinions upon one of the prob- 
lems still confronting us today; namely: the determination of the 
most advantageous spacing and depth for lateral drains. 

The earliest tile drainage in the United States was probably that 
of John Johnson of New York. While on his way to the coast to 
leave Scotland, Mr. Johnson saw some tile being burned, and on mak- 
ing inquiry was so impressed with the idea that he brought some 
of" the tile to this country with him. He laid his first tile in 1835. The 
results were so gratifying that he continued the work till in 1851 he 
had 16 miles of tile drains. In the meantime, he imported a tile 
moulding machine, after which tile could be obtained at a price, which 
as he expressed it, left the farmer no excuse for wet land. It seems 
probable that Mr. Johnson laid his tile lines very close together and 
it is known that he used very small tile (not to exceed two inches in 
diameter) with collars. 

With the westward emigration from the New England States 
to Ohio and Indiana, and later from there to Iowa and Illinois, came 
the practice of tiling the wet lands of the fertile Mississippi Valley. 
There has been a constant and rapid progress in this work during the 
last quarter of a century till now these two states, and particularly 
Iowa, have taken the lead in underdrainage development. The knowl- 
edge of the forms and movements of soil moisture, or groundwater, 
and their relationships to underdrainage and to crop production has 
increased greatly, and the engineering features of underdrainage de- 
sign have been made more accurate through investigations which 
have developed new methods and formulas, or shown how to use 
old ones more correctly. 



3. Purpose of this Bulletin. The general knowledge of the rela- 
tionships between soil characteristics, the forms and movements of 
soil moisture, underdrainage and crop production is so small that 
many engineers and landowners either consider these factors as al- 
most wholly unrelated or do not consider the possibility of an inter- 
relationship at all. In reality these factors, in so far as they are of 
especial significance in underdrainage work, are all closely inter-re- 
iated and inter-dependent. 

Some readers may feel at first that much of the material presented 
in this bulletin is an encroachment on the sphere of the agriculturist, 
but such is not the case. The underdrainage systems of Iowa are 
constructed primarily in agricultural lands to increase the potential 
production of those lands. The study of soils and soil moisture and 
their relation to crop production is essentially agricultural, but the 
relation of these to drainage is a part of drainage engineering and a 
knowledge of them is essential to the engineer employed in under- 
drainage work. 

The question of the most advantageous spacing and depth for 
underdrains, regarding which James Smith and Joseph Parks held 
different views nearly a century ago, is not debated so much now as 
it is ignored by landowners and engineers who unthinkingly follow 
some precedent or local custom. The question of the proper rate of 
runoff is treated in much the same manner and is usually decided 
upon as an independent feature of the design, and this decision 
altered to meet the landowner's ideas as to the amount he thinks his 
drainage system should cost. The proper spacing and depth for 
lateral underdrains are dependent upon the amount of the surplus soil 
moisture and the rate of its movement through the soil. These 
in turn are dependent quite largely, in each locality, upon the 
character of the soil. The same principles which determine the 
proper spacing and depth for the lateral underdrains also determine 
the rate of runoff from these drains, and a correct understanding of 
these principles is dependent upon a general knowledge of soils and 
soil moisture. 

It is the purpose of this bulletin to present those principles which 
determine the efficiency of the operation of a well-constructed under- 
drainage system. Only those phases of the whole study of soils and 
soil moisture that are essentially a part of drainage engineering are 
discussed, and these are treated from a drainage standpoint. It is not 
intended that any part of this bulletin be taken as a discussion of 
these principles from a strictly agricultural standpoint. 

Some additional matter of a more strictly drainage engineering 
nature has been included so as to make a more complete presentation 
of the principles governing the design of underdrainage systems. 

It is hoped that these discussions will be of value both to the engi- 
neer and to the landowner and that they will lead to a more general 
and correct understanding of the principles of the intelligent design 
of underdrainage systems. 



4. Acknowledgements. Much of the material in this bulletin, par- 
ticularly those chapters relating to soils and the forms and movements 
of soil moisture, is a compilation of material gathered from other 
sources. These subjects and their relation to crop production come, 
primarily, in the field of the agriculturist ; the discussions of these 
points are taken principally from agricultural text books. The only 
claim for originality in this connection is in the presentation of the 
whole subject from a drainage standpoint. 

The suggestions and criticisms offered by Mr. A. Marston, for- 
merly Dean and Director of the Division of Engineering, Mr. W. H. 
Stevenson, Professor of Agronomy and Mr. S. W. Beyer, Dean and 
Director of Engineering, all of the Iowa State College, have been of 
much value in preparing and arranging the subject matter presented. 
Acknowledgement is also made of the valuable assistance given by 
Professor Stevenson and Mr. R. E. Smith, Associate Professor of 
Soils, in reviewing those chapters relating to soils and soil moisture. 

II. SOILS. 

A clear understanding of the nature and movements of soil water 
as related to both agriculture and drainage requires some knowledge 
of the various kinds of soils and of their physical or mechanical com- 
position. Such elementary phases of the subject as are thought 
necessary for a comprehensive study of the succeeding discussions 
will be presented here. 

5. Soil Areas of Iowa. The typical soil areas of Iowa might be 
listed as follows: 



Wisconsin Drift. 

Iowan Drift. 

Southern Iowa Loess and Kansan Drift. 

and (5) Missouri and Mississippi Loess. 

Moraines. 

Gumbo. 

Bottom Lands or Alluvium. 

Peat. 



a: 

(2: 

(3: 
(4: 

(6^ 

(7) 

(8) 

(9) 

Each of these different soil areas has its peculiar properties and 
each presents a different problem in drainage due, primarily, to dif- 
ferences in topography and the physical or mechanical composition of 
the soil. The map (Fig. 1) showing the location of the six larger 
areas and the description of each area are taken in most part from 
published reports of the Iowa Agricultural Experiment Station. The 
data as to the peat areas are taken from the published report of in- 
vestigations by the State Geological Survey. 

(1) Wisconsin Drift Area. The soil of this area is the till deposited 
by the Wisconsin Glacier, the last of the great Keewatin ice-sheets 
to invade Iowa. This drift area includes all or part of 30 counties in 







W^m&0£& 





10 

the western portion of the north-central part of the state. This area, 
geologically speaking, is very new and its natural drainage very in- 
complete. A few large streams, notably the Des Moines River and 
its large tributaries, cross it but have not had time to extend their 
tributaries very far back from the main streams. Nearly the whole 
area is a wide prairie with numerous sloughs and ponds, except for 
the eastern and western edges where stretches of low hills, the re- 
mains of terminal moraines, occur. 

The soil of practically all of this area is a black loam top soil, 
sometimes of a sandy or clayey nature, with a yellow clay or sandy 
yellow clay subsoil. A few small areas of partially decayed peat are 
found in the swales and sloughs. Owing to the newness of this drift 
the rocks are only partially decomposed causing the formation of light 
alkali spots around the edges of a few ponds and sloughs which have 
surface drainage but not' adequate underdrainage. Nearly all of 
this area requires, and much of it now has, artificial drainage, both by 
underdrains and open ditches. Thorough underdrainage has been 
found to be a very efficient remedy for the alkali trouble. 

(2) Iowan Drift Area. The Iowan Ice-sheet invaded Iowa at an 
earlier period than the Wisconsin, and extended considerably further 
east, so that the Iowan Drift Area now exposed covers an area just 
east of soil area No. 1. The Iowan Drift Area comprises all or part of 
29 Counties. This area is nearer maturity than the Wisconsin Drift 
Area and has more complete natural drainage, as is evidenced by the 
absence of ponds and sloughs, and by the deeper and more pronounced 
natural water courses. 

The soil of the Iowan Drift area is much the same as that of the 
Wisconsin. The top soil is generally a black loam, sometimes sandy 
or gravelly, while the underlying strata are sand, sandy clay or clay. 
Some sections of this area have a substratum of sand or gravel at a 
depth of four to six feet which often is a great aid to underdrainage, 
though this layer is quite often overlain with a layer of clay or sandy 
clay 6 inches to 18 inches in thickness. Practically no peat and but 
very few "alkali spots" are found in this area. 

(3) Southern Iowa Loess and Kansas Drift Area. This area is 
shown in Fig 1 as the Southern Iowa Loess Area, and includes prac- 
tically all of the state south of the Wisconsin and Iowan Drift Areas, 
between the Missouri and Mississippi Loess Areas. The subsoil for- 
mation is of the till from the great Kansan Ice-sheet which covered 
Iowa long before either of the other ice-sheets mentioned. At a later 
time the deposit known as the Iowa Loess was placed over this deposit 
of till. This area is much older than either of the other drift areas 
and has numerous well defined natural drainage channels, though 
many nearly flat areas of both bottom land and upland now exist. 

The top soil of this area is the loess deposit and may be described 
as a fine black loam. It is usually found now only on the divides, 



11 

erosion having removed it from the hillsides. Where found upon the 
fairly level divides it usually has a depth of about two feet. The sub- 
soil is the till from the Kansan Glacier, and upon the badly eroded 
portions this former subsoil is now the top soil. This till is a very 
close clay, usually yellowish red to red brown in color. It is so im- 
pervious as to have given the name "Hardpan Area" to much of this 
section. Neither the loess nor the till underdrain as readily as the 
soils in other portions of the state, though the efficiency of the under- 
drainage systems increase with the length of service. 

(4) and (5) Missouri and Mississippi Loess Areas. These two 
areas are similar so far as the characteristics of interest in this dis- 
cussion are concerned, and hence are classed together. The location 
and extent of these areas are shown by Fig. 1. These areas contain 
many examples of the two topographical features characteristic of 
deep loess deposits ; namely, the well-rounded convex curves of the 
slopes and ridges, and the vertical escarpments. Although this soil 
erodes easily, it still has the property of standing nearly vertical in cuts, 
either natural or artificial, as is evidenced by the steep bluffs along 
the Missouri River. 

The soil of these loess areas is rather porous and loose. It often 
allows of such rapid aeration that the humus is decayed so rapidly 
that the desired amount cannot be accumulated. From an agricultural 
standpoint, this area is marked by infertile tracts often of as much 
as 50 acres, the infertility being accredited to the loss of the surface 
layer of soil. The loess is not unusually as much as 100 feet deep, 
so that it can be readily underdrained. 

(6) Moraines. The areas of morainal drift now exposed in Iowa 
lie in narrow belts on the eastern and western edges of the Wisconsin 
Drift Area and two small areas within this drift area. These areas 
were formed principally at the extremities of the ice-sheet and in con- 
sequence have better developed systems of natural drainage channels 
and a correspondingly smaller need for underdraining. 

The soil of these morainal areas is very similar to that in the ad- 
joining drift areas, except that it contains a larger percentage of 
coarse material and more boulders. 

From the standpoint of underdrainage these morainal areas are 
comparatively unimportant. Because of the development of the nat- 
ural drainage systems only those areas of bottom land along the 
streams are wet enough to need underdrainage. 

(7) Gumbo Areas. The Gumbo areas of Iowa are a very small por- 
tion of the whole state and are found in only two localities. A narrow 
strip from two to five miles wide, extends from the center of Wood- 
bury County to the center of Pottawattamie County, parallel to the 
Missouri River and about seven miles east of it. In Washington, Mus- 
catine, Henry, Des Moines, Van Buren and Lee Counties are sev- 



12 



eral irregular but small areas. Gumbo is found in both upland and 
lowland areas, and is practically the same, from a drainage stand- 
point, wherever found. 

The soil of these gumbo areas is very fine grained and waxy, and 
is usually black in color. The lowland gumbo is usually finer grained 
than that of the uplands, though the latter is often just as sticky 
and waxy as the former. 

Strange as it may seem, this soil seems to drain more readily than 
the black loam soils of other areas. Reports show that underdrains 
placed as much as 200 feet apart have given satisfactory results, and 
where ever underdrains have been laid in Iowa gumbo, there is re- 
ported to be little doubt that lateral drains 100 to 150 feet wide will 
give satisfactory results. 

8. Bottom Lands or Alluvium. These terms are applied to the soil 
formations in the present and former flood plains of streams. Owing 
to the mode of formation the characteristics of these soils vary 
greatly. The top soil is usually a fine black loam ; the subsoil may be 
anything from a very close clay to a coarse gravel, though each of the 
separate layers is usually of fairly uniform composition and fairly dis- 
tinct. 

If the subsoil is of close clay, as often occurs, this soil is more 
difficult to drain than any of the other types of Iowa soil, unless it be 
those sections of Southern Iowa where the close clay of the Kansan 
Drift is at or near the surface. It can be successfully underdrained, 
however, if the proper spacing and depth of drains be used. 

(9) Peat. The peat areas of Iowa may be divided roughly into two 
classes ; the thick deposits found in the morainal belt along the east- 
ern side of the Wisconsin lobe, and the shallow beds in the counties 
to the west. 

The peat beds in the morainal belt vary in thickness, when wet, 
from three or four feet to twenty or thirty feet and occasionally more. 
The peat in the shallow deposits further west is rarely over three 
feet thick, even when wet. The subsoil of the peat areas is almost 
universally a marly clay. 

The Report of the Iowa Geological Survey, Vol. XVII, gives the 
areas of the peat beds of Iowa, by Counties, as follows : 



Clay 

Dickinson 

Emmet - 

Hamilton 

Hancock 

Kossuth 



Trace 

50 acres 
400 acres 
.Trace 

375 acres 
625 acres 
Franklin 



Palo Alto - 
Webster 
Winnebago 
Wright 
Worth 

Cerro Gordo • 
590 acres 



Trace 

800 acres 
4055 acres 

800 acres 
1540 acres 
1610 acres 



Owing to the impurities in these peats they are generally avail- 
able for agricultural uses within two or three years after they are 



13 

thoroughly drained. Tile drainage is not a difficult matter wherever 
it is possible to place the tile in or upon the subsoil. The instability 
of the wet peat, and its great shrinkage upon drying make under- 
drainage of the deeper peat beds uncertain because of construction 
and maintenance difficulties. 

6. Mechanical Composition of Soils. Viewpoints of the Drainage 
Engineer and the Agriculturist : The agriculturist in considering the phys- 
ical properties of the soil of any region will divide his information 
into that relating to the top soil, or the seed-bed of cultivatable crops, 
and that relating to the subsoil. The drainage engineer may or may 
not divide the soil in this way depending upon the thickness of the 
top layer of soil. If this surface layer is four or five feet thick the 
lower stratum will have little effect upon the underdrainage unless its 
perviousness or imperviousness be extreme. The drainage engineer 
will look rather upon the soil as a whole, and base his conclusions 
upon its general physical properties, considering of course any un- 
usual formations. In some cases he may need to consider more than 
two layers or strata. The drainage engineer should not ignore the 
study of the agricultural possibilities of the soil in any proposed drain- 
age district because upon this will usually depend the feasibility, from 
an economic standpoint, of the proposed work. 

Texture and Structure: The physical character of a soil is usually 
expressed by the terms texture and structure. The term texture is used 
to refer to the size, or range in sizes, of the individual soil grains, while 
the term structure refers to the arrangement of these particles. 

7. Soil Texture. Soil Separates: The various classes of soils, ordi- 
narily designated as clay, loam, sandy clay, etc., derive their physical 
characteristics and textural classes and groups from the size of the 
individual grains of which each is composed. The size of grain which 
places a soil in one of these classes is chosen arbitrarily, though those 
adopted by the United States Bureau of Soils are usually taken as a 
standard. The soil whose grains, as determined by analyses, are all 
between the upper and lower limits of size set for one class, is termed 
a soil separate. The list of the soil separates and the limits of sizes of 
grains for each as adopted by the Bureau of Soils are given in Table 1. 

TABLE I. SOIL SEPARATES. 

Diameter of Grains, 
Separate Millimeters 

Fine gravel 2.0 - 1.0 

Coarse sand 1-0 - 0.5 

Medium sand JJ-"}_ " 0-^5 

Pine sand 0.25 - 0.10 

Very fine sand 0.10 - O.Oo 

gilt 0.05 - 0.005 

Clay ".'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'...... - 005 " °- 00 

(1 milimeter = 0.03937 inch.) 

Soil Separates in Common Soils: These soils separates must not be 
confused with the ordinary soil groups, (clay, loam, sandy loam, etc.) 
even though the names are similar. Each of the various soils, as 



14 



popularly designated, is made up of varying percentages of the soil 
separates given in Table I. Tables II and III show the percentages 
of the various separates present in some of the common soils together 
with some of their physical properties. All the data in Table III, ex- 
cept the per cents of pore space and the numbers of soil grains per 
gram, are the results of analyses of samples of these soils. 

TABLE II. 
PHYSICAL COMPOSITION OP COMMON SOILS. 





1 

• ~ ijt 

Fine 
Gravel 


2 

Coarse 
Sand 


3 

Medium 
Sand 


.4 

Fine 
Sand 


5 
Very 
Fine 
Sand 


6 
Silt 


7 
Clay 


Coarse 


More than 25% 
(1 4- 2) 








0-15% | 0-10% 


Sand 


More than 50% 
(1 + 2 + 3) 


Less than 20% 
(6 + 7) 


Medium 
Sand 


Less than 20% 
(1 + 2) 








0-15% | 0-10% 




More than 20% 
(1 + 2 + 3) 


Less than 20% 
(6 + 7) 




Less than 20% 
(1 + 2 + 3) 






0-15% | 0-10% 


Fine 
Sand 


Less than 20% 
(6 + 7) 




More than 20% 
(1 + 2 + 3) 










10-35% | 5-15% 


Sandy 
Loam 


More than 20% 

and less than 50% 

(6 + 7) 




Less than 20% 
(1 + 2 + 3) 






10-35% | 5-15% 


Fine 

Sandy 

Loam 


More than 20% 

and less than 50% 

(6 + 7) 






- ■ 








1 15-25% 


Loam 


Less than 
55% 






More than 50% 
(6 + 7) 


Silt 
Loam 








\ 


More 1 Less 
than 55%|than25% 










1 


25-55% | 25-35% 


Clay 
Loam 


More than 60% 
(6 + 7) 


Sandy 




1 


1 

1 

1 


Less | More 
than 25%|than 20% 


Clay 


I^ess than 60% 
(6 + 7) 


Silt 
Clay 










More |25%-35% 
than 55% | 










1 




More 
than 35% 


Clay 






More than 60% 
(6 + 7) 



Taken from "Principles of Soil Management," Lyon and Fippin. 



15 









TABLE III. 












PHYSICAL, 


COMPOSITION OP 


COMMON SOILS. 
























Surface 


. 


Per cent 


of Separate Present 


N 




Number of Area 










Very 








Grains in 


of 1 


Fine 


Coarse 


Medium 


Fine 


fine 






Pore 


1 gram, 


Cu. Ft., 


Soils — gravel 


sand 


sand 


sand 


sand 


Silt 


Clay 


Space 


millions 


sq. ft. 


Coarse Sand ...5.0 


13.0 


27.0 


30.0 


11.0 


8.5 


5.5 


40.0 


3,276 


40,500 


Medium Sand ..5.0 


13.0 


20.0 


32.5 


14.0 


9.0 


6.5 


41.8 


3,956 


44,500 


Sandy Loam ...5.0 


10.0 


11.0 


26.0 


11.0 


22.0 


15.0 


51.0 


6,485 


66,600 


Fine Sandy Loam 2.0 


2.5 


5.0 


20.0 


27.5 


32.0 


11.0 


50.0 


4,902 


62,000 


Silt Loam 1.0 


1.5 


2.5 


6.0 


11.0 


56.0 


22.0 


53.0 


9,639 


104,000 


Clay Loam . . . .3.0 


5.0 


5.0 


12.0 


10.0 


28.0 


37.0 


54.0 


16,371 


136,000 


Clay 0.0 


2.0 


2.5 


5.5 


7.0 


37.0 


46.0 


56.0 


19,525 


142,000 



(Compiled from data given in "Principles of Soil Management," Lyon & Pippin.) 

Soil Separates and Crop Adaptation: Figure 2 shows the crop adapta- 
tion and composition of some soils whose analysis gave the data from 
which the curves were plotted. By watching the yields of various 
crops and noting the general characteristics of any soil and then re- 
ferring to these curves it should be possible to determine something 
of its physical composition. 




5 

Very Fine 

oand 

Fig. 2. Crop Adaptation and Composition of Some Soils. 



However, it should be borne in mind that these data, both as to 
physical composition of the soil and the crop to which it is best 
adapted, refer more particularly to the seed bed or surface layer than 
to the total depth that must be considered in connection with under- 
drainage. 

Size of Soil Grains: The sizes of the individual particles which com- 
pose any natural soil vary greatly. For example, the sandy loam listed 
in Table II has particles of fine gravel, which may vary in size from 



16 

2.0 m.m. to 1.0 m.m. and the clay, whose particles are less than 0.005 
m.m. in diameter. If the largest limit be taken for the clay particles, 
it would require 5050 of them to span one linear inch. The variation 
in sizes of the grains composing all soils is not as great as that just 
referred to, though the variation is great for all natural soils. 

Effective Diameter: All formulas for the movement of water through 
the soil take into account either directly or indirectly, the size of the 
grains composing the soil in question. Where the size of grains is 
used directly it is expressed as the "effective diameter" or the "effec- 
tive size." This is a diameter such that if grains of this diameter 
were substituted for the soil in question the porosity would remain 
the same. The effective diameter is also defined as the diameter 
of the grain which has 10% of the grains smaller than itself and 90% 
larger. In other words, the effective diameter or effective size of 
grain in any soil, however determined or defined, is a size such that 
if grains of this size were substituted for the soil in question a soil of 
the same hydraulic properties, as far as the movement of ground- 
water is concerned, would be produced. 

Uniformity Coefficient: In his formula for the movement of water 
through filter sands, Mr. Allen Hazen introduces a factor known as 
the "Uniformity Coefficient" in order to give expression to the variety 
of sizes of the grains of any sample. To determine this coefficient, 
first find the size of grains such that 60% of the material is of smaller 
grains and 40% of larger grains. This result divided by the effective 
diameter of grain for the entire sample gives the uniformity coeffi- 
cient. In his determination on filter sands, Mr. Hazen made a sieve 
analysis, and then plotted a curve using the diameter of grains as 
absessae and the per cent by weight passing through the various 
sieves as ordinates. From this curve it was easily possible to loca'te 
the effective size of grain and the size which divided by the effective 
size would give the uniformity coefficient. 

It is doubtful if, with the present methods of determining the size 
of soil grains, it would be possible to apply this method to natural 
field soils. The grains in natural soils are too small to permit of suc- 
cessful sieve analysis so that it is probable that the best method of 
determining the effective size and uniformity coefficient would be to 
determine the per cents of the various soil separates present and then 
compute the value of the effective size and the uniformity coefficient. 
Such a result would be only approximate at best and for calculating 
soil water movement would probably not be desirable. 

In his book on Soil Physics, Professor King gives the data in Table 
IV for samples of the various soils. It will be noticed that there is 
a little apparent variation in the data in this table and that in Table 
III, but this is probably due to slight differences in the samples of 
the various soils analyzed. 





Surface 


Per 


Area of 1 


Cent of 


Cu. ft. of 


Pore 


Soil Grains, 


Space 


sq. ft. 


52.94 


173,300 


48.00 


110,500 


44.15 


91,960 


49.19 


70,500 


47.10 


53,490 


44.15 


46,510 


38.83 


36,880 


34.45 


15,870 


34.91 


8,381 



17 

TABLE IV. 

PROPERTIES OP COMMON SOILS. 

Effective 
Size of 
Soil Grains, 
Kind of Soil. M. M. 

Finest Clay Soil 004956 

Fine Clay Soil 008612 

Heavy Red Clay Soil 01111 

Loamy Clay 02542 

Clayey Loam 01810 

Loam 02197 

Sandy Loam 03035 

Sandy Soil 07555 

Coarse Sandy Soil 1432 

Number of Soil Grains: The number of grains in a sample of soil 
may be readily computed when the mean diameter of the grains and 
the specific gravity of the soil particles are known. The value of 
2.65 for the latter is fairly constant for the soil particles, but it is not 
correct for a volume of the soil. The data as to the numbers of soil 
grains in 1 gram of soil given in Table III were computed in this 
way from data obtained from an analysis of each sample. 

Surface Area of Soil Grains: The surface area of the particles com- 
posing a given weight of a soil may be computed from the number 
of particles and their mean diameter, each particle being taken as a 
true sphere. The relation of the fineness and number of particles to 
the surface area is well illustrated by the data in Tables III and IV. 
The finer the grains the greater is the number of particles and the 
total surface area of particles. This fact has a very important bear- 
ing on the capacity for, and movement of, capillary moisture. 

8. Soil Structure. The term structure is used to refer to the ar- 
rangement or grouping of the soil particles. Upon the structure de- 
pends, to a large extent, the per cent of pore space and the size and 
number of pores. This effect will be treated more fully under a dis- 
cussion of porosity. 

In any soil in natural condition, especially the top or cultivated 
layer, the individual particles are found not as simple grains but as 
groups of grains or granules. If these grains be arranged very com- 
pactly in the granules the effect is to produce a soil which has the 
same properties in regard to the movement of water and air as a 
larger grained soil. Arrangement No. 4 of Fig. 3 is an illustration of 
the effect of such a grouping into granules. Some soils have varying 
sized grains which become mixed as do the particles in a well mixed 
concrete. The smaller particles fill the spaces between the larger ones 
so completely as to make it a nearly compact mass. When many of 
the fine-grained soils are subjected to long continual saturation by 
standing water, the finer particles move downward till they lodge 
in the soil pores. This action produces a very compact layer which is 
often termed "hard pan." When a soil, which is composed of grains 
of properly graded sizes, is worked while wet, as by the tramping of 



18 

stock, the smaller particles are carried into the spaces between the 
larger grains. This produces a condition commonly known as 
"puddled." 

9. Porosity. Porosity and Size of Soil Grains: The relation of the 
soil grains to the percentage of pore space is well shown by the data in 
Tables III and IV. The clay soils having the smallest sized grains 
have the largest per cent of pore space, while the sands and gravels 
having the larger sized grains have small per cents of pore space. This 




Fig. 3. Ideal Arrangement of Spherical Soil Particles. 
(1) and (2), columnar order, 47.64% of pore space; (3) oblique order, 25.95% 
of pore space; (4), compound spheres in oblique order, 74.05% of pore space; 
(5), three sizes of spheres with closest packing, about 5% of pore space. 



may lead to the question of why the sandy soils appear 1 to be more 
porous than the clays. It is well known that a sandy soil will drain 
out much more readily than a close fine-grained clay soil. This ap- 
parent contradiction is due to the fact that pore space in the clay soil 
is divided up into such minute channels that water or air passes 
through them with difficulty, while in the sandy soil with its smaller 
amount of pore space the pores or channels are much larger and 
afford a much freer passage for either air or water. The effect of the 
amount of pore space and size of pores will be discussed more fully 
in the following chapters. 

Porosity and Structure: The porosity of any soil depends not only 
upon the size but also upon arrangement of the individual particles. 
The principle is well illustrated by the data in Tables III and IV and 
by Fig. 3 which shows the effects of arrangement if all particles be 
considered as spheres. Arrangements 1 and 2, Fig. 3, have the same 
per cent of pore space but in the first arrangement there are only 16 
pores while in the latter there are 64. The spheres in these two cases 
have diameters of 1 and J / 2 respectively. If the diameter be again 
reduced so that it is % the per cent and total amount of pore space 
remains the same but the number of pores is increased to 256. _ The 
effect of this reduction upon the movement of soil water or soil air 
is readily seen as in the first case it is divided into only 16 streams 
while in the second case it is divided into 64 streams, each propor- 
tionately smaller than those of the first arrangement. If the particles 
be arranged in the oblique order shown as Arrangement 3, the pore 



19 

space is reduced to a minimum for spheres of uniform size. But if the 
structure be that shown in Arrangement 5, the pore space is further 
reduced because the smaller particles continually fill the spaces be- 
tween the larger ones. The effect of the grouping into granules is 
shown in Arrangement 4, which gives the largest per cent of pore 
space of any of the arrangements shown. 

Figure 3 must not be taken as illustrating the arrangements of par- 
ticles in any natural soils. These diagrams are only given to show the 
effects of different arrangements upon the total pore space and the 
size of pores. The particles in any natural soil would not conform to 
any of these arrangements but would be modifications and combina- 
tions of all or of several of them. 

10. Porosity and Perviousness. Because of a misunderstanding of 
their true meaning, these two terms are often used as synonyms. 
These terms are neither synonyms nor antinyms, though they are 
more nearly antinyms than synonyms ; in a number of soil samples the 
most porous, the soil with the highest per cent of porosity, is normally 
the least pervious. 

The porosity of a soil depends upon the total amount of pore space, 
and has no reference to the size or hydraulic efficiency of the channels 
formed by these pore spaces. The perviousness, or permeability, of a 
soil depends upon the readiness with which liquids or gasses (usually 
water or air) will pass through the soil. This depends upon both the 
size and the hydraulic efficiency of the channels formed by connected 
pores. Normally, the soil with the smallest individual grains has the 
highest per cent of pore space. In such soils, as very fine grained 
clay soils, the soil pores are so small and are so disconnected that the 
soil is practically impervious. On the other hand, the coarser grained 
soils, as sand or granular loam soils, have a much smaller per cent 
of pore space, but the pores are larger and form such efficient hydraulic 
channels (comparatively) that soil air and soil water pass through 
them rapidly. 

In loam soils, porosity is determined largely by the soil texture 
and the permeability by the soil structure; the porosity normally de- 
creases as the permeability increases or vice versa. 

III. SOIL MOISTURE. 

In discussing soil moisture and its movement only such phases of 
the subject as have an especial relation to underdrainage will be con- 
sidered. The subject in its entirety is so complex that only an ele- 
mentary treatment is either possible or desirable here. 

11. Source of Soil Moisture: Of the water which falls to the 
earth's surface as rainfall, one portion runs away over the surface to 
the natural drainage channels, another portion seeps into the ground, 
and the third portion evaporates from the place where it falls. It is 



20 

this second portion, that which is taken up by the soil, which is of 
particular interest in this discussion. 

12. Moisture Content of Soils: The capacities of soils to take up 
and hold moisture vary greatly with the different soil types and 
formations. The soils with the larger percentages of porosity hold 
the larger amounts of moisture. The moisture content is usually ex- 
pressed as a percentage of either the dry weight or of the volume of 
soil, or as a depth in" inches, which is simply a modified way of ex- 
pressing it as a percentage of the volume. Table V shows the varia- 
tion of the moisture capacity of a soil with the fineness of _ the soil 
grains. The maximum and minimum moisture content each increases 
as the size of the soil grains decreases. Any treatment, such as 
underdrainage, as will tend to increase the moisture capacity of a 
soil without too great injury in some other respect is desirable. 

TABLE V. 

MOISTURE CAPACITY OP SOILS. 

Amount of Available Water 

Depth, 

Water Capacity Cu. in. inches 

Min. Max. per in top 4 

per cent per cent Per cent cu. ft. ft. of soil 

Light Sandy Loam Early Truck Soil 3 8 5 122 3.4 

Silt Loam Bluegrass Soil 15 25 10 218 6.0 

Clay Black Cretaceous Prairie Soil 23 40* 17 274 7.6 

(From "The Principles of Soil Management," Lyon & Fippin.) 

Moisture Content and Crop Production: The amount of moisture 
necessary for crop production varies greatly with the soil texture and 
structure, the crop and the climate. Lyon and Fippin state in their 
text, "The Principles of Soil Management," that " — other things being 
equal, more water will be required in an arid region than in one of 
humid climate ; more in a warm region than in a cold region ; more in 
a clay soil than in a sandy soil ; more in a windy section than in a 
region of still atmosphere; more with a high soil-moisture content; 
more on a poor soil ; and lastly, more water is used per pound of dry 
matter produced in a small crop than is required in a large crop. Not 
only is the total seasonal requirement to be considered, but the maxi- 
mum demand of the crop at any period of its growth must be met." 

Investigations by Hunt and by King show that one acre of corn 
in Illinois will, in July, take 1.55 inches of water from the soil in one 
week. This indicates the large amount of water required by this 
one crop. The other crops common to Iowa agriculture, unless it be 
grasses, all require more water than does corn for the total crop 
growth. 

Professor King of the University of Wisconsin has determined that 
the average crop under the best management requires water equiva- 
lent to 3.7 to 15 inches of rainfall for the average yield. From Table 

'Assumed. 



21 

V it is seen that the surface four feet of even the clay soil does not 
hold quite one-half enough moisture for some crops, while the other 
two soils hold even less. This means that for an average yield the 
average crops require that the moisture content of the soil shall be 
replenished during the growing season. 

13. Forms of Soil Moisture. Soil moisture is of three different 
classes: (1) Gravitational water, or that which is free to move under 
the influence of gravity ; (2) Capillary or film moisture, which is held, 
by surface tension, against the influence of gravity; and (3) Hygro- 
scopic moisture or that which condenses from the atmosphere upon 
the surface of the soil particles. Fig. 4 shows the proportional 
amounts of these forms of moisture in a soil and the availability of 
each for plant use. 

Hydroscopic Capillary Gravitational 



<0| 



~~\( ^ r \ 



J\. 




Unavailable Available Injurious 

Fig. 4. Proportional Amounts of the Three Forms of Soil Moisture.* 

14. Gravitational Moisture. Land drainage has been aptly defined 
as the removal of the surplus moisture from the soil. (Usually this 
is only the means to an end, as the benefits of drainage are due to 
those actions made possible by the removal of the surplus moisture). 
Underdrainage is the removal of this water by artificial or natural 
means under the surface. As it is only the gravitational soil water 
which is free to move under the influence of gravity, and which is 
unavailable for and injurious to plant growth, the need for drainage 
is proportional to the amount of this form of moisture present in the 
soil. 

When the supply of soil moisture is replenished by rainfall that 
part in excess of what can be held capillarily becomes gravitational 
moisture. As the water percolates from the surface downward the 
thickness of the films of capillary moisture near the plane of satura- 
tion is gradually increased till the whole pore space is filled. None of 
the soil moisture becomes gravitational till after the full capillary 
moisture capacity has been supplied. This of course takes no account 
of the relatively large amount of water which passes from the satu- 
rated surface through the shrinkage cracks, small root cavities and 
worm bores. 

Gravitational Moisture Content of Soils: The gravitational water- 
content is directly proportional to size of the spaces and is also the 
difference between the total moisture content and the capillary and 
*From "The Principles of Soil Management," Lyon and Fippin. 



22 

hygroscopic moisture contents. If the pore spaces become too small 
they may be almost entirely filled by capillarity as in the fine grained 
clay soils. In general it may be said that the gravitational water 
capacity decreases as the total amount of pore space increases, because 
the largest total percentage of pore space is ordinarily found in the 
soil having the smallest grains and the smallest individual pore spaces. 

TABLE VI. 
MOISTURE CONTENT OP SOILS. 

Max. Gravita- 

Weight Maximum Maximum tional Capacity 

Per Cubic Pore "Water Capillary Depth, 

Foot, Space, Capacity, Capacity, inches in 

Kind of Soil. Pounds per cent per cent per cent Per cent top 4 ft. 

Dune Sand 80 52 40.5 10.7 29.8 18.3 

Coarse Sand 81 51 39.5 10.6 28.9 18.0 

Pine Sandy Loam 83 50 38.0 18.0 20.0 12.7 

Silt Loam 83 50 38.0 20.9 18.9 12.1 

Clay 68 59 54.5 30.4 13.9 7.3 

Muck Soil 15 80 333.0 250.0 83.0 9.6 

Note.- — Percentages are figured by weights. (From "Principles of Soil Manage- 
ment," Lyon & Fippin.) 

The above principles are well illustrated by the data in Table VI. 
It is seen here that the gravitational water capacity varies directly as 
the porosity until the clay is reached. In this fine grained soil the 
pore spaces are so small as to be more nearly filled by capillary mois- 
ture. The quantities given in the columns headed "Max. Gravitational 
Capacity" are the amounts which might be removed by drainage. In 
considering the data in the columns headed, "Max. Gravitational Ca- 
pacity," it should be borne in mind that these values are for a saturated 
soil and represent the maximum amounts of water which are sub- 
ject to removal by drainage from each of these soils. If an acre of 
the "Dune Sand" be saturated to a depth of over four feet, the water in 
this top four feet would be equal to 18.3 in. in depth over the entire 
acre. This amount is that which a drainage system should carry 
away in order to restore this acre of land to the best condition. 

Some determinations were made of the percentage of porosity of 
the soils, in natural field condition, of the fields at Hanford, Cerro 
Gordo County, Iowa, to which a part of the original data in Bulletin 
No. 52 refers. 

This value of the porosity as given by these determinations, is very 
nearly equal to the gravitational water content of these soils, as they 
had all or part of their maximum capillary water content when the 
determinations were made. The results of these determinations are 
summarized in Table VII. 

TABLE VII. 

GRAVITATIONAL WATER CAPACITIES OF SOILS NEAR HANFORD, CERRO 

GORDO COUNTY, IOWA. 

Gravitational 

Depth Water 

of Sample Capacity, 

Kind of Soil. Below Surface Per cent 

Black Top Soil 3 in. 18.5 

Clay, a little sand 2.5 ft. 5.8 

Whitish Yellow Clay 3.5 ft. 2.5 

Blue Clay 4.5 ft. 6.6 

Sandy Clay 3.0 ft. 13.9 

Sandy Clay 4.0 ft. 18.6 



23 

A.s is shown in Fig. 4 only a very small part of the gravitational 
moisture in the soil is available for plant use, and the major portion 
of it is injurious to vegetable life. Below the plane of saturation it 
completely fills the pore spaces in the soil, thus excluding the air. 
Many authorities on agriculture contend that aeration, or the passage 
of air through the soil, is one of the most important factors of crop 
production as far as it is controlled by the condition of the soil. 

Watcrtable: The surface of the gravitational water in the soil, or 
the surface of the saturated layer, is commonly called the watertable. 
It is also referred to as the groundwater level, groundwater in this 
sense meaning gravitational soil moisture, or surplus moisture. 

It sometimes happens that the presence of air in the soil causes two 
planes of saturation. After a rain there are sometimes a saturated 
surface layer and a true groundwater level at a greater depth. The 
pore spaces of the intermediate layer of soil are filled with air which 
excludes the water till such time as the air can pass out through the 
upper saturated layer. Figure 5 illustrates this condition as shown by 
an experiment by Professor King. The arrows indicate the movement 
of both the water and the air ; as soon as the surplus water in the 
saturated surface layer has worked down to the readjusted true water- 
table, the surface of the latter will not curve upward toward the ex- 
perimental well as is shown in the diagram. Under the conditions 
illustrated the water is passing from the well to the lower saturated 
layer, hence the upward curve of the w r atertable near the well. 

Between periods of rainfall the movement of capillary soil mois- 
ture is from the watertable upward. In this one particular, that of 
furnishing a source of supply for capillary moisture, the gravitational 
moisture is very beneficial. 

15. Capillary Moisture. In so far as plant life is concerned this is 
the most valuable form of soil moisture and, in fact, the only form 
which is available for the sustaining of plant growth. It is held, against 
the force of gravity, in the small pore spaces between the soil grains 
and as a thin film surrounding each individual particle or group of par- 
ticles. Every one has noticed the rise of water in a small bore glass 
tube when the lower end is immersed in water, the height to which the 
water rises increasing as the size of the opening in the tube decreases. 
It is the same force, surface tension, which holds the capillary water 
in the soil. 

Capillary-Moisture Content. In the field the grains of soil are sur- 
rounded by connecting thin films of moisture and the finer the soil 
particles the greater the surface area which holds this film of mois- 
ture. This variation of the capillary moisture capacity of soils with 
different sized grains is illustrated by the data in Table VI, already 
referred to. 

An idea of the amount of water held in this film, and the thickness 
of the film, may be obtained by considering the fine clay soil where 



24 



"LTcSuperscrt^ura : tec/.\ '■ 




Zone of Soil . 

• » • * . 

_ ■ oir wh/cn . 
• * • - . » * 

• s\ • • - * 

t " /i jr escapmq\ .* 

» - - . * . ■ * 



m 



ff- Surface So//>~±: 



with confined ../ • 



impedes peroolahon. ■ 



, * . /a?/o jve// ' . * 



• » * 




Fig. 5. Diagram Illustrating the Condition Obtaining After a Rain When the Soil 
Pores Are Filled With Air. 



25 

the effective diameter of soil grains is 0.005 m.m. or less. Professor 
King has determined (See Table IV) that for 1 cubic foot of such soil 
the area of the surface of the soil grains is 173700 sq. ft. or approxi- 
mately four acres. Water equivalent to four inches in depth over 
one square foot could be held in one cubic foot of this soil if the thick- 
ness of the film was 173,000 inches, or about one-half the thickness 
of a soap bubble just before it expands to the bursting point. 

Available Capillary Moisture. However, not all of even the capillary 
moisture is available for plant use. A certain portion of it is held so 
intimately that the small roots cannot draw it from the soil. This 
fact is illustrated by the data in Table VIII, which is compiled with 
data given in Lyon and Fippin's "The Principles of Soil Manage- 
ment." The values in the column headed "Approximate % of Mois- 
ture at which Crops will Wilt," are the amounts of water which are 
held so intimately in the soil as to be unavailable for plant use. The 
sum of the values in this column and those of either the succeeding 
columns represent very approximately the total combined capillary 
and hygroscopic water capacities of those soils to which they refer. 

TABLE VIII. 

AVAILABLE MOISTURE IN SOILS. 

Approximate Available Moisture 

Dry Per cent of Depth 

Porosity, Water Crops inches in 

Kind of Soil. Percent Will Wilt Percent top 4 ft. 

Dune Sand 52.2 3.0 7.7 4.6 

Coarse Sand 51 3.0 7.6 5.2 

Fine Sandy Loam 50 5.0 13.0 8.5 

Silt Loam 50 10.0 10.9 6.9 

Clay 59 17.0 13.4 7.0 

Muck Soil 80* 80.0 170.0 20.5 

Capillary Moisture and Drainage. Capillary moisture has no direct 
relation to underdrainage though it has a very definite and important 
bearing upon the results and effects of drainage. When the plane of 
saturation is lowered by the use of underdrains the moisture necessary 
for crop production must be supplied by capillarity. 

16. Hygroscopic Moisture. This is the least important of the 
three forms of soil moisture and in amount it is relatively very small. 
It is the moisture which condenses from the air onto the soil grains, 
the condensation taking place either at the surface or in the- soil, if 
it be so open as to allow of a circulation of air. 

Hygroscopic-Moisture Content. The amount of this form of mois- 
ture in a soil is a function of the surface area exposed and conse- 
quently is greater in the finer grained soils. The amount also varies 
inversely as the temperature in the soil. Table IX shows the amounts 
of hygroscopic water held by three soils, each of which was a soil 
separate obtained by careful analysis. 

*Assumed. 



26 

TABLE IX. 

PER CENTS OF HYGROSCOPIC MOISTURE AT 21°C., OR APPROXIMATELY 70°F. 

"Very fine sand 18 

Silt 7.3 

Clay 16.5 

Taken from "The Principles of Soil Management," Lyon & Fippin. 

Comparison of these data with others in the text from which these 
were taken show that all soils hold a considerably larger percentage 
of capillary water than of hygroscopic water. The importance of this 
form of moisture is further reduced by the facts that it is varying in 
amount and unavailable for plant use. It has practically no bearing 
at all upon the underdrainage of the soil. 

17. Loss of Soil Moisture. The total amount of moisture in the 
soil at any one time is a function of the supply and the loss. The sup- 
ply may be either from rainfall or from percolation from higher 
ground; the loss will be from percolation, evaporation or transpiration 
of plants. Relatively the loss by percolation is much larger than 
either or both of the other two. As a general rule it may be said that 
where the soils are such that percolation is very slow the losses by 
evaporation will be high comparatively. 

The loss due to percolation naturally has to do with the movement 
of gravitational water so will be discussed under the head. 

The losses due to evaporation are in the nature of thermal move- 
ments and will be discussed under that head. 

IV. SOIL-WATER MOVEMENTS 

18. Soil-water Movement and Drainage. The relations of the dif- 
ferent movements of soil water to underdrainage are the same as those 
of the forms of soil moisture to which they refer. The gravitational 
movement, or the percolation of the groundwater, is the one by which 
the surplus water reaches the drain, and is, consequently, the most 
important in this connection. Just as capillary moisture has an im- 
portant relation to the effects of underdrainage, so has the capillary 
movement, as it is the effectiveness of this movement which deter- 
mines whether or not the underdrained soil shall be constantly sup- 
plied with the moisture necessary for plant growth. 

19. Forms of Soil-water Movement. The previous discussion of 
the types of soil water will readily suggest the forms of soil-water 
movements. Each of the three forms of soil water has a distinct type 
of movement. These movements are : (1) Gravitational, or the move- 
ment of gravitational water under the influence of gravity ; (2) Capil- 
lary, or the movement of capillary moisture due to capillarity or sur- 
face tension ; and (3) Thermal, or the movement due to changes in 
temperature. This latter movement is in the form of water vapor, 
though changes in temperature have some effect upon the other two 
types of movement. 



27 

These three types of soil-water movement will be discussed in the 
reverse of the order in which they were stated. Because of its very 
important and direct relationship to underdrainage the discussion of 
gravitational movement will be left till the last then be given more at- 
tention than the other two. 

20. Thermal Movements. This movement takes place as the 
movement of water vapor and consequently is relatively very small. 
If the bottom of a column of soil be heated, the cold portion directly 
above the heated portion will be found to have added moisture. The 
moisture from the heated layer has been converted into water vapor 
and as such has passed up through the heated soil to the cold portion 
where it has been recondensed. If none of this soil column had been 
cold enough to condense this vapor it would have passed upward and 
outward to the air. Such an action would have resulted in the loss of 
soil moisture by evaporation. 

Evaporation. Evaporation of soil moisture may take place not only 
at the surface as is popularly supposed but also in the deeper pores 
of the soil. When a dust mulch is maintained the evaporation is very 
apt to take place at the surface of the moist layer. The loss due to 
this movement depends upon the rate at which diffusion into the 
atmosphere takes place. Buckingham found that the loss due to evap- 
oration from sandy, silty, or clayey soils where the porosity was near 
50% varied from 4.3 inches per year for the sandy soils to about 1 
inch per year from the clayey soils. In the ordinary agricultural 
soils of Iowa this loss is so small as to be of very little importance in 
a study of soil moisture as related to underdrainage. 

21. Capillary Movement. In the discussion of capillary soil mois- 
ture, its importance to plant life and its relation to underdrainage were 
explained. Its movement naturally bears the same relation to these 
as the moisture itself. It is of vital importance to crop production 
but of very little importance to the operation of underdrainage. 

Form of Movement and Factors Governing It. The form in which 
capillary moisture occurs in the soil has been explained as a thin film 
of moisture surrounding the individual soil particles. Its movement 
then is a film movement and depends upon an unbalanced condition 
in the pull exerted upon these films. Any moisture which is moved 
in this way naturally moves as a film and consequently is small in 
amount. This movement always takes place from the wetter soil to 
the dryer regardless of the direction. 

The soil factors governing this movement are texture, structure 
and dampness. The finer the soil particles the more surface is ex- 
posed and consequently the greater capillary pull is exerted. In gen- 
eral it may be said that the extent of capillary movement is inversely 
proportional to the fineness of the soil grains. It is also true, that 
when the soil grains are very fine the spaces between them become so 
very minute and the thickness of the film of moisture so reduced as 



28 

to so increase the friction that the movement is very slow. It is also 
necessary for water to wet any substance to which it is held by capil- 
larity, hence if the soil particles are somewhat dry the moisture is not 
held to them strongly until after they become damp, as all natural 
field soils have a certain resistance to wetting'. 

The rate and extent of this movement are also affected, to some 
degree, by the temperature changes. The surface tension is greater 
for low temperatures than for high, causing both a greater rate and 
a greater extent of movement at the lower temperatures. On the 
other hand, warm water is more limpid than cold and will conse- 
quently pass through the small pore spaces more readily. 

Relation of Capillary Movement to Under 'drainage. The most im- 
portant phase of this movement in relation to underdrainage is the 
distance which a sufficient quantity of water for crop production will 
be raised. It is the effect of any drainage to lower the permanent 
watertable after which the moisture for sustaining plant life must be 
furnished capillarily. If the watertable is held at too low a stage an 
insufficient amount of moisture will be raised to the upper soil layers 
and the crops may suffer. In such a case the underdrainage system 
will have been too effective and its results will be no better than those 
conditions which obtained before any drainage was attempted. 

Rate and Extent of Capillary Movement. As has been stated the rate 
and extent of capillary movement depend upon the distance the water 
is raised and the nature of the soil through which the movement takes 
place. In Part II of the 19th Annual Report of the United States Geo- 
logical Survey is given experimental data from which Table X was 
compiled. These data illustrate the principle that the rate of move- 
ments varies inversely with the extent and distance of the movement. 
The lower rate of movement in the clay soil from each lift is probably 
due to the greater friction and greater resistance to movement in the 
finer grained soil. 

table x. 

POUNDS OF WATER PER DAY PER SQUARE FOOT OF SOIL RAISED FROM 

DIFFERENT DEPTHS. 

■ Depths 

Soil. 1 foot 2 feet 3 feet 4 feet. 

Medium Fine Sand 2.37 2.07 1.23 0.91 

Medium Clay Loam 2.05 1.62 1.00 0.90 

The report referred to above states further that though there were 
very few reliable data on this subject those at hand indicated that for 
natural field soils the movement is fairly rapid when the lift does not 
exceed four feet. If the rate given for that lift in Table X be main- 
tained throughout the year a quantity of water equivalent to 63.8 
inches in depth would be raised. Experiments already referred to 
show that corn in Illinois used water during July equivalent to a depth 
of 1.55 inches or 1.15 pounds per square foot per day. When this 
moisture is to be supplied by capillarity, as is the case during periods 
between rains, the lift should not exceed 2.5 to 3.0 feet, if the crop is 



29 

to be fully supplied. It has also been previously stated that for the 
average yield under the best methods of cultivation the average crop 
requires from 3.7 to 15 inches in depth of moisture or from 19.24 to 78 
pounds of water. If the average growing season be taken at 80 days 
this would mean that from .24 to .96 pounds of moisture would be 
required per day per square foot. This data in Table X indicates that 
to meet this demand the lift for capillary moisture should be about 4 
feet. However, the amount of moisture needed by a crop varies so 
greatly with stage of its growth and with the season that the above 
figures can be only approximate. 

Capillary Movement and Depth of Undcrdrains. From the standpoint 
of the supply of capillary moisture the above data indicate that the 
ideal underdrainage system for the average Iowa soil which is to be 
used for the ordinary field crops is one which will maintain the 
groundwater level at a depth of from 2.5 to 3.0 feet below the average 
elevation of the crop roots. However, this optimum depth varies 
slightly for the various crops and soils. This depth seems to afford 
the maximum storage capacity without so lowering the watertable as 
to reduce the capillary movement too greatly. There are, however, 
other factors than the supply of capillary moisture which must be 
considered in determining the depth of underdrains. 

22. Gravitational Movement. Form of Movement: It was stated in 
the discussion of gravitational soil water that the need for drainage 
was directly proportional to the amount of this form of moisture pres- 
ent. Consequently, the nature and rate of this movement will be a 
very important factor, and in most cases the deciding factor, in the de- 
sign of the underdrainage system. It is true that the optimum posi- 
tion of the watertable with relation to capillary movement should be 
considered, but it is probable that the rate at which the surplus, or 
gravitational, water is removed will be of greater importance. 

The gravitational movement is the result of the gravity pull upon 
the soil water, and, as both the capillary and the hygroscopic mois- 
ture are held against gravity, it is only the gravitational soil moisture 
which is affected. This moisture and its movement are usually re- 
ferred to as "groundwater" and "groundwater movement" or "perco- 
lation." 

The direction of this movement depends entirely upon the soil 
formations. Being a gravity movement the direction is always as 
nearly vertically downward as the soil conditions will permit. From 
the previous discussions of soil composition and formations it will 
be readily seen that the channels afforded by the soil pores will not be 
straight and where the percolating water encounters a more or less 
impervious layer the direction may be so changed as to have only a 
very slight slope ; or if the percolation is through a porous layer be- 
tween two nearly impervious layers which are undulating or which 
approach the surface closely, the movement may be upward, as- in 
the case of the outlet to a spring. 



30 

Channels for Movement. In all natural field soils the percolation of 
the groundwater, especially downward from a saturated surface layer, 
occurs in two ways ; through the soil pores and through the shrinkage 
cracks, small root cavities and worm bores. The movement through 
the soil pores is subject to both theoretical and experimental investi- 
gation. The flow through the comparatively large channels afforded 
by the shrinkage cracks, etc., is indeterminate. The presence of these 
openings can be detected in the soils of any field, but the amount of 
channel capacity thus afforded could be determined only by a great 
number of careful mechanical analyses. The rate of flow through 
these larger openings is comparatively great because they afford a 
much more efficient channel. 

The following discussion of groundwater movements will be con- 
fined entirely to the flow through the soil. In a later discussion of 
runoff from underdrained areas the total flow through the soil, both 
through the soil and through the large channels mention, will be con- 
sidered. 

The flow of water through the channels afforded by the pores of 
the soil is subject to the same laws as the flow through a pipe, a tile 
drain or an open ditch. In each case the velocity of flow depends upon 
the slope and the resistance which the sides of the channel offer to the 
passage of water. The rate of flow is the product of the velocity and 
the cross-sectional area of the moving stream. 

The size, shape and hydraulic efficiency of the soil-water channels 
depends upon the texture and structure of the soil. However, while 
these are the only soil factors influencing the movement of gravita- 
tional soil water, some external forces have a definite influence upon 
the rate of this movement. 

Effect of Temperature and Barometric Pressure. Changes in temper- 
ature affect the rate of movement both through the effect upon the 
air and upon the moving water. Likewise the barometric pressure 
tends to increase or decrease the rate of flow as the barometer rises 
or falls. The wind may cause a suction or an increased pressure at 
the outlet, the flow being effected accordingly. 

The exact amount of the affect of temperature upon the rate of 
percolation of soil water, except for the effect upon the water itself, is 
indeterminate, though it may be approximated by experimentation. 
A lowering of the temperature of the soil air will cause it to contract 
causing a suction which tends to hold the soil water. An abrupt 
warming of the soil air will have the opposite effect. Consider, for ex- 
ample, the conditions obtaining in a well underdrained soil. In the 
evening the outer air cools much more rapidly than that in the soil. 
This cooler air is the denser and flows to the more rarefied and 
warmer air in the soil. When the cool outer air passes from the tile 
into the soil, the soil air cools and contracts causing a suction which 
tends to hold the soil water. In the morning the outer air warms 
more rapidly than the soil air, and due to the circulation within the tile 
and the soil, the soil air is warmed, causing it to expand and tend to 



31 

force out the soil water. In the 19th Annual Report of the United 
States Geological survey are given some records of the variation in 
the rate of discharge from a tile drain due to the change in tempera- 
ture during the day. When the rates of discharge and temperature 
are plotted as curves they are found to be approximately parallel, the 
rate of discharge increasing during those hours when the tempera- 
ture should be expected to increase and decreasing towards evening. 
It should be borne in mind, however, that these changes were due 
to the effect of temperature change upon both the soil air and the outer 
air and upon the water itself. 

The variation in the rate of discharge from a tile drain or a spring, 
due to barometric changes, is very similar to that due to temperature 
changes. The rate of flow increases and decreases as the barometer 
rises and falls. The changes in the pressure in the outer air are 
transmitted through the tile to the soil air causing a suction or pres- 
sure as it contracts or expands. In the same publication referred to 
above are given curves showing the change in rate of discharge from 
underdrains and springs and the barometric changes for the same 
periods. In each case the curves are nearly parallel except that the 
changes in the barometric pressure occur more rapidly than the 
changes in rate of discharge. Professor King found from these records 
that the effect of changes in barometric pressure were sufficient to 
vary the rate of discharge as much as 8% for springs and as high as 
15% for underdrains. 

The changes in rates of groundwater movement due to tempera- 
ture and barometric changes are interesting rather than valuable in a 
discussion of underdrainage. The changes due to each are usually 
comparatively small and are compensating. The present knowledge 
of the movement of water in the tile and of other factors of drainage 
engineering is not sufficiently accurate to demand any changes in the 
design of an underdrainage system to care for the variations from 
these causes. It is probable, however, that for extremely large under- 
drained areas this variation in the rate of flow would be noticeable, 
especially if accurate discharge records were kept. 

Effect of Soil Texture. The influence of the texture of a soil upon 
the movement of groundwater is due to the variation in the size of 
the soil pores resulting from differences in the sizes of soil grains. 
The finer the soil grains are the smaller the pore spaces and, conse- 
quently, the higher the resistance which they offer to the passage 
of percolating water. The rate of percolation varies directly as the 
size of the soil grains until in some very fine grained soils the pores 
become so small as to be completely filled with capillary moisture. 

As has been stated the movement of gravitational soil water is gov- 
erned by the same laws as the flow through a pipe under gravity head. 
In the pipe the velocity of flow is determined by the slope and the re- 
sistance which the pipe offers against the passage of water. This re- 
sistance, considering only that within a straight pipe, is due to the 
friction between the water and sides of the pipe and so varies directly 



32 

as the wetted area of pipe surface. From the laws of the variation of 
the area and circumference with the diameter, it is readily seen that 
the resistance in a small pipe is proportionally much greater than that 
in a large pipe. 

The application of the above principles to the movement of water 
through soils may be illustrated by considering Fig. 3 as representing 
sectional views of soils and considering each grain as a cylinder of 
unit length and of the same diameter as the spherical soil grains. If 
the diameters of cylinders arranged as shown in Arrangements 1 and 
2 of Fig. 3 be taken as 1 inch and ]/$ inch, respectively, the cross-sec- 
tional area of the pore spaces will in each case be 3.4336 square inches, 
while the surface area of these pores will be 50.2556 square inches and 
201.0224 square inches respectively. In other words, decreasing the 
diameter 4 times leaves the total cross-sectional area of the pore space 
the same but increases the surface area 4 times and, if the frictional 
resistance be taken as the same per unit of area in each case, the re- 
sistance is 4 times as large with the small grains as with the larger. 

The above discussion is of general principle only, when applied to 
soils in natural field condition, because there the grains are not ar- 
ranged in any such regular order and are not of uniform size. But 
the principle that the ratio of the exposed surface area to the cross- 
sectional area of the pore space increases at a more rapid rate than 
the diameter of soil grains decreases holds true, though this ratio may 
vary considerably. It is these ratios which will determine, to a 
large extent, the ratios of the flows through different soils. Every one 
has noticed how much more quickly the water left upon a loose sandy 
soil will disappear than that left upon a field of close grained clay 
soil. This is only an illustration of the above principles. 

Effect of Structure. The effect of different arrangements of soil 
grains upon the rate and extent of the movement of groundwater is 
governed by the same principles as that referring to the effect of var- 
ious sized soil grains. If the arrangement is such as to give a large 
exposed surface area in proportion to the cross-sectional area of the 
pore space the flow is small. If the same assumption be made as in 
considering the soil grains of 1 inch and % inch diameters in the 
preceding paragraph, grains of 1 inch diameter and arranged as in Ar- 
rangements 1 and 3 of Fig. 3, the cross-sectional area of each pore will 
be .2146 square inches and .04 square inches respectively. In other 
words, the area of the pore space for 1 inch grains arranged in 
columnar order is 5 times that for the same sized grains arranged in 
oblique order. The areas of the exposed surfaces in these two cases, 
considering each grain as a cylinder of unit length, are 3.1416 square 
inches and 1.5758 square inches respectively. The ratio of the ex- 
posed surface to the area of the pore is 14.6 for the columnar arrange- 
ment and 39.4 for the oblique order. It is thus seen that with the 
oblique arrangement the pore space is both smaller and less efficient 
than in the columnar arrangement. 



33 

In the section on "Soils" the property of some soils to form 
granules, or groups of grains, was discussed. As far as the percolation 
of groundwater is concerned, a soil of granular structure is a large 
grained soil, and, as such, allows a much more rapid groundwater 
movement than it would if composed of the individual grains. There 
are some small pore spaces within the granules themselves but these 
are so small as to allow of only a very small flow in comparison to that 
passing through the pores between the granules. 

The Viscosity of the Moving Water. The effect of temperature upon 
the movement of gravitational water is of considerable importance. 
The viscosity of water decreases as the temperature rises, and, conse- 
quently, the flow increases with the temperature of the water. Hazen 
found in his experiments with filter sands that the relative flow at 
different temperatures varied from .70 at 32° to 1.0 at 50° and 1.45 at 
77° . All temperatures were measured in degrees fahrenheit. In other 
words, raising the temperature from 32° to 77° approximately doubles 
the flow. This fact more than anything else, accounts for the variation 
in the flow from tile drains. However, the temperature within the 
underdrained soils varies much less than that of the air. It is prob- 
able that during the growing season in Iowa the temperature of 
drained soils is very nearly constant at 50° fahrenheit. At least, if 
temperature were to be taken into account directly in computing the 
flow through Iowa soils, it would seem safe to make such calculations 
with a soil temperature of 50° F. 

23. Do Tile Drains "Draw?" How far will a tile drain "draw?" 
The statement that the underdrains in one field "draw" better, or 
farther, than those in another field is often heard. Actually the drains 
do not draw at all, if by "draw" it is meant that the tile exert a pull 
lending to suck the water out of the soil pores and into the drain. 
The underdrains serve simply as a collecting channel, or outlet, for the 
percolating water. If one field is drained farther back from the line of 
tile than another field it signifies simply that the conditions as to 
depth and capacity of the tile, or of the soil are such as to cause a more 
leady movement of the groundwater in the one case than in the other. 

How Water Moves from the Surface to the Drain. It is the opinion 
of many persons, who have given the subject little serious considera- 
tion, that the water falling upon the surface of an underdrained area 
moves in a diagonal direction till it is directly over the tile and then 
enters it from the top. This is not the case, as no water ordinarily en- 
ters the tile from the top except that which falls upon the surface di- 
rectly above the drain and then only when the soil conditions are such 
as to allow of a nearly vertical percolation. 

The path taken by a drop of water passing from the surface to the 
drain is illustrated by Fig. 6. The surplus water in the surface layers 
passes as nearly directly downward as soil conditions will permit until 
it reaches the watertable, and then moves laterally so as to enter the 
drain from the bottom. Its movement after reaching groundwater 



34 



level is somewhat uncertain, but the paths illustrated in Fig. 6 are 
the general paths of this movement. The water below the plane of 
saturation is under a certain hydraulic or hydrostatic head and fol- 
lows the path of least resistance, which, in this case, is into the drain. 
The only case when water generally will enter the drain from the 
top is when the tile is or has been surcharged and the soil above has 
become saturated. 



Surface of Ground 



i.i , .1 



I 




Impen/iou5 Stratum 



Fig. 6. Diagram Illustrating the Movement of Water from the Surface to the 

Drain. 

24. Natural Underdrainage, Seeps and Springs. An area is said 
to have natural underdrainage when there is a subsoil stratum of sand 
or gravel at a depth of from 2' to 6' below the surface. In such area 
the surplus water from rainfall may pass down through the top soil to 
this sand layer and through it to an outlet. The thoroughness of such 
drainage depends upon the readiness with which the surplus water in 
the top soil layer reaches the layer of sand and the rate at which it 
passes away through this sand layer. 

However, such lands, especially if they are very flat, usually re- 
quire artificial underdrainage if they are to be efficiently drained. If 
the top soil is fairly loose and porous and if the said substratum is 
not at too great a depth, say from 4 to 5 feet, the drains may be placed 
at the top of the sand stratum and at considerably greater distances 
apart than if placed in the top soil. Care must be taken in such design 
to see that the tile are well supported whenever placed in any sand 
stratum. 



35 

Every one has noticed seepy or spouty spots on the sides or at the 
foot of the steeper slopes. These wet places are especially evident 
where there is a porous layer overlying a stratum of very close clay, 
both of which outcrop upon the slope. The surplus soil water works 
down into the porous layer and then follows it to the outcrop, that 
being the only outlet. In some few cases these wet places are caused 
by the flow from a porous layer which is both overlain and underlain 
by impervious layers. In this case the soil water enters the porous 
layer at a point higher up the slope where the overlying impervious 
layer is very thin or above the point of its beginning. 

The drainage of these wet spots is accomplished by tracing the 
porous stratum for a few feet back from the outlet and then construct- 
ing intercepting tile drains. In some instances the drains may be laid 
at a sufficient depth in the wet areas so as to intercept the seep water 
as it rises to the surface. 

Springs, as they are ordinarily considered, are not usually a factor 
to be considered in the design of underdrainage. The location is gen- 
erally such that the flow will not interfere with any cultivatable lands 
and, as the flow is more often than not a fairly constant quantity, pro- 
vision for them can be made without difficulty. 

The character of the soil formation causing springs is very similar 
to that causing the second class of "spouty" spots mentioned in a 
preceding paragraph. In cases where a spring causes conditions de- 
manding relief by underdrainage, an intercepting drain is necessary 
if the flow is to be prevented from coming to the surface. 

25. Rate of Groundwater Movement. Several investigators have 
endeavored to express the laws governing the flow of water through 
soil, or other porous media, in a mathematical formula. Such formulas 
contain constants or factors representing the character of the soil 
(as the fineness of soil grains, and the porosity), the length and cross- 
sectional area of the column of soil through which the flow occurs, the 
loss of head in passage and the temperature, or the viscosity, of the 
moving water. In one formula all of these factors may be represented 
by separate quantities while in others one or more of the soil factors 
and the temperature factor may be represented by a constant. 

These formulas, in their original form, are unsuited for application 
to drainage work because of their great refinement. To be practically 
and readily usable a formula for drainage uses will require no data 
which are not easily obtainable and will not be more accurate than 
the original data or the other formulas which will be used in the de- 
sign of the system. The effective size of soil grain for a given sample 
of natural soil can be determined only by a careful laboratory analysis, 
which is both impractable and undesirable in drainage design. Such 
an analysis would give data of a much greater degree of refinement 
than the other data which will be used in the design. Much the same 
thing is true also of the determination of the porosity, if the absolute 
value is to be determined, though the space available for the move- 



36 

ment of the surplus moisture can be determined without great diffi- 
culty. 

Darcy's Formula.One of the earliest attempts to determine, and state, 
the laws of the flow of water through soils was made in 1856 by Darcy, 
a Frenchman. He announced that the flow of water in a certain 
direction through a column of soil is proportional to the difference in 
pressure at the two ends of the column and inversely proportional to 
the length of the column.* Darcy expressed his conclusions in the 

P 

formula v = k — 

h 
where v=the velocity of the moving water. 

p=the difference in pressure at the two ends of the column. 
h=the length of the column. 

k=a constant whose value depends upon the character of the 
soil, especially the size of the soil grains, and which must 
be determined experimentally. 

The principles expressed by Darcy has been incorporated in all 
subsequent formulas for the movement of groundwater. In reality, 
Darcy did no more than say that, other things being equal, the rate 
of flow was proportional to the hydraulic grade which is true of the 
movement of water under all conditions. 

Mr. Allen Hazen, Consulting Engineer, of New York, has derived 
a formula for the movement of water through filter sands. His 
formula is the same as Darcy's with added factors representing the 
effective size of soil grains and the effect of temperature. According 
to Mr. Hazen's statements his formula is true for only sands with 
grains within certain limits of size, the lower limit being much larger 
than the grains of natural field soils. 

Mr. Charles S. Slichter of the United States Geological Survey 
has derived a theoretical formula for the movement of water through 
porous media. As far as is known, Mr. Slichter's formula* is the only 
purely theoretical investigation of this subject which has ever been 
published. His theoretical derivation was made in conjunction 
with an experimental investigation by Prof. F. H. King, of the Uni- 
versity of Wisconsin. In some trials, the flow through certain soils 
was computed by Mr. Slichter and measured in the laboratory by Mr. 
King. These results gave such close checks as to prove that this 
formula was correct if used for soils which had been accurately an- 
alyzed. This formula requires so careful a laboratory analysis of the 
soil through which the movement is to be calculated as to prohibit its 
use in drainage work. 

26. Relation of Groundwater Movement to Underdrainage. Since 
only gravitational moisture, or groundwater, is free to move under 
the influence of gravity, this is the only moisture which can be re- 

*U. S. Geological Survey, Water Supply Paper No. 67. 



37 

moved by the drains. The rate of its movement through the soil to 
the drains is the controlling factor in determining the efficiency of the 
drainage. The primary function of the underdrains is to make pos- 
sible this movement by providing an outlet for it. 

According to Darcy's principle, the velocity of the movement of 
water through a particular soil depends upon the hydraulic slope of 
the moving water. It follows then that any change in the lateral 
system which causes a change in this hydraulic slope will cause a 
corresponding variation in the velocity of the movement of the ground- 
water that is flowing through the soil to the drains. As the rate of 
this movement determines the efficiency of the drainage, it follows 
that the lateral system should be so designed as to give the desired 
control of this movement, or of the rate of removal of the surplus 
moisture. In other words, the spacing and depth of the laterals in a 
particular field controls the rate of the movement, and of the removal 
of the surplus moisture, and this in turn determines the volume of 
flow which determines the capacity required in the mains and sub- 
mains. 

The application of Darcy's principle to the design of underdrainage 
systems may be summarized in the. three statements given below. 
Each of these is based upon the premise that all factors, except that 
stated as a variable, remain constant. From the nature of the prob- 
lem, it naturally follows that the converse of each of these statements 
is equally true. 

(a) The rate of the movement of the groundwater to the drain varies in- 
versely with the distance between the laterals. 

(b) The rate of this movement varies directly with the depth of the under- 
drains (actually varies with the vertical distance from the crest of the 
watertable to the water in the drain). 

(c) The rate of runoff from the underdrainage system varies directly with 
the rate of the movement of the groundwater to the drain. 

The first two of these principles will be discussed in the following 
paragraphs. The relationship of the spacing and depth of laterals 
to the rate of runoff will be taken up in succeeding articles. 

Effect of Spacing of Laterals. Darcy's principle states that the ve- 
locity of water moving through any given soil varies directly with the 
hydraulic slope of the moving water. (In this connection the hy- 
draulic slope is the quotient obtained by dividing the vertical distance 
from the crest of the watertable, or plane of saturation, to the free 
surface of the water in the drain by the horizontal distance between 
these two points.) This means simply that if the distance between the 
laterals be decreased, other conditions remaining the same, the 
hydraulic slope of the moving groundwater is increased and that it 
will move to the drain at a more rapid rate. This will tend to remove 
the surplus moisture at a more rapid rate which, in turn, means that 
the soil is in the condition required for proper plant growth a larger 
percentage of the time. This increase in the rate of the movement of 



38 

groundwater also requires that the mains and submains of the sys- 
tem be designed to care for larger flows. Suppose, for example, that 
the drains in a particular field are 100 feet apart. If intermediate 
drains were constructed so that laterals would be only 50 feet apart, 
the rate of flow to the drains, for any position of the watertable, would 
be just twice that when the drains were 100 feet apart. 

The form of the surface of the plane of saturation, or the water- 
table, has already been explained. Fig. 7 illustrates the condition just 
described. The lines D and E represent two positions of the water- 
table when only drains A and C are constructed. After the inter- 
mediate drain B has been constructed, the positions of the watertable 
are illustrated by lines F, G, H and I. It can be seen from this dia- 
gram how the hydraulic slope of the water moving to the drain is 
made steeper by decreasing the distance between the laterals. 

This diagram also illustrates the way in which the closer laterals 
cause an increase in the area of soil which is free from surplus mois- 
ture. The importance of this latter fact is emphasized by the fact 
that plant roots can neither live nor draw food from a saturated soil. 





A B C 

Fig. 7. Diagram Illustrating Effect of Closer Spacing of Laterals. 



The form of the watertable between drains and the rate of move- 
ment of groundwater will vary with differing soil conditions. The 
relationship discussed above cannot be applied directly to systems in 
different fields. If the soil conditions are similar approximately cor- 
rect comparisons can be made. This principle is of more value in 
considering the various systems that might be used in one field and in 
explaining the general proposition of the relative efficiencies of dif- 
ferent underdrainage systems. 

Effect of Depth of Laterals: The discussions just given will explain 
the relationship between the depth of laterals and the rate of move- 
ment of the surplus soil water. Few, if any, underdrainage systems 
are so designed and constructed th^t the plane of saturation will not 



39 

rise to the surface during or after heavy rains. It is apparent then, 
that if the soil is uniform, that placing the laterals deeper will cause 
an increase in the rate of movement of surplus moisture as will a de- 
crease in the distance between laterals. However, other considera- 
tions control the depth of laterals to within such close limits that this 
relationship is of less importance than that of the spacing in laterals. 

In considering the effect of placing the laterals at a greater depth 
the effect of the variation in the soil at different depths must be con- 
sidered. For example, if the subsoil is a very close clay, the move- 
ment of groundwater though this stratum will be much slower than 
through the more open top soil. This increase in the resistance to the 
flow may be sufficient to more than counterbalance the increase due to 
the steeper hydraulic grade. In some soils, such as were referred to 
above, drains only two feet deep may appear to be just as effective, 
or in extreme cases, more effective, than drains four feet deep. A 
greater depth than this is desirable in order to increase the depth of 
soil that is in condition to furnish plant food. This close subsoil us- 
ually contains valuable plant food which must be made available if 
the field is to produce the maximum crops. The close soils become 
more open and drain more readily after the drains have been in opera- 
tion for a time, and experience shows that this improvement continues 
as time goes by. The solution of the problem presented by such a 
field is to place the laterals closer together and hold to the four foot 
depth. 

It will be readily seen from Fig. 6 or 7 why the average position 
of the watertable is lowered as the drains are placed deeper. The 
drains cannot lower the plane of saturation below their own level, and, 
practically, cannot hold it to their own level during the growing sea- 
son when rains are usually frequent. The plane of saturation must 
have an appreciable slope (the hydraulic slope upon which depends 
the rate of flow to the drain) toward the drain, or the rate of move- 
ment of the soil waters will be so slow as to be of small consequence. 
The food requirements of the crops and the plant food generally avail- 
able then demand that drains be not less than four feet deep except in 
unusual cases. In these discussions no consideration has been given 
those cases where the depth of the drains may be controlled by con- 
struction features, this discussion being confined to the relationship of 
the depth of the drains to the rate and extent of the removal of the 
surplus soil moisture. 

The Time Factor: The relationship of the spacing and depth of lat- 
erals to the rate of the movement has been explained. The increase 
in this rate which results from certain changes in the lateral system 
has an important bearing upon the crop production and upon the cost 
and degree of thoroughness of the cultivation. In each soil, each posi- 
tion of the watertable represents a certain amount of surplus mois- 
ture. If changes in the lateral system cause the removal of this sur- 
plus moisture at a more rapid rate, the loss due to this surplus is cor- 



40 

respondingly, reduced. If the surplus moisture is removed more rap- 
idly there is a larger average depth of soil available to support the 
growing crop. The removal of the surplus moisture at a more rapid 
rate allows the field to be prepared and the crop to be planted earlier. 
The more rapid removal of the surplus after each rain allows the field 
to be cultivated with less loss of time because the ground is too wet 
for proper cultural operations. In short, the more rapid removal of 
the surplus moisture means a decrease in the total time during which 
the plane of saturation is too high to permit of the maximum plant 
growth and proper cultural operations. This point is of especial im- 
portance when the underdrained area is to be used for truck crops. 

5. RUNOFF. • 

27. Definitions. The portion of the rainfall or snowfall which 
passes over or through the ground to the natural or artificial drainage 
channels is termed runoff. It is that part which passes through the 
soil to the drains which is of particular interest in this discussion. 

The rate of runoff is variously expressed as a percentage of the 
rainfall, as the depth in inches removed from the watershed per 24 
hours, and as cubic feet per second, or per 24 hours, from each unit of 
watershed area. 

It is probable that the method most common in present day drain- 
age practice is to express the rate of runoff as the depth in inches 
removed from the watershed in 24 hours. This depth is often re- 
ferred to as the runoff coefficient or the drainage coefficient, or in the 
abbreviated form, of a "^-inch runoff." This method of expressing 
the rate of runoff will be used in this bulletin. 

28. Factors Affecting Runoff. The relationship of the form of 
the lateral system to the rate of the movement of the surplus moisture 
through the soil has been explained. Since the runoff from an under- 
drainage system is the surplus soil moisture which flows through the 
soil and collects in the drain, it follows that the rate of runoff is di- 
rectly dependent upon the spacing and depth of the laterals. The 
efficient operation of the underdrainage system demands that it be 
so designed that it will remove the surplus moisture at a sufficiently 
rapid rate; if this is not done the primary object of the construction 
is not attained. The climatological, topographical and physical fea- 
tures which determine the amount of rainfall concentrated on the area 
to be underdrained and the soil characteristics which determine the 
rate of movement through the soil, jointly determine the form of the 
lateral system necessary to secure the desired control of the ground- 
water. Those factors which are ordinarily considered to control the 
rate of runoff should be considered to control the type of the lateral 
system and then the probable rate of flow from each system should 
be determined as accurately as possible and used as the rate of runoff 
for which the capacities of the various lines of the system are designed. 



41 

The principal natural or physical factors controlling the amount 
and rate of movement of surplus soil moisture are : rainfall, topog- 
raphy of the watershed, character of the soil, evaporation and trans- 
piration of plants and seasons and climate. The general relation of 
each of these to the form of the lateral system and its resulting effect 
upon the rate of runoff will be discussed in the above order. 

Rainfall. Since rainfall is the source of supply of soil moisture, the 
amount of surplus moisture to be removed by the drains naturally 
varies with the rainfall. In sections where the rainfall is heavy, a 
proper control of the soil water will demand a closer lateral spacing, 
these conditions resulting in a high rate of runoff. The total annual 
runoffs from two underdrainage systems, of similar size and design, 
draining similar watersheds so located that the annual precipitation is 
30 inches and 60 inches, will be approximately in the ratio of one to 
two. 

However, the total amount of surplus moisture to be removed dur- 
ing the year is of very much less importance in this connection than 
is the probable maximum amount which must be removed per unit of 
time in order to secure proper soil conditions during the cropping 
season. Both the ordinary maximum daily precipitation and the rate 
at which it falls must be recognized as controlling factors. If the 
underdrainage system is to be constructed in a locality where the 
daily rainfall is ordinarily heavy during the cropping seasons, but 
consists of severe short showers, as thunder storms, the water will 
reach the earth much more rapidly than it can be taken up by the soil. 
Under such conditions much of the rainfall must necessarily flow away 
over the surface, with the result that there will be but a compara- 
tively small amount of surplus moisture to be removed by the under- 
drains. The rate of underdrainage runoff due to a rain of one inch 
in one hour will often be much smaller than that due to a rain of one 
inch falling, slowly but steadily, in a period of six to twelve hours. In 
the latter case more nearly the total amount of water falling will pass 
down into the soil and on to the drains. 

The effect of hard dashing rains was well illustrated by the records 
from two adjoining drainage systems in Southern Iowa, where the 
Engineering Experiment Station was conducting an investigation dur- 
ing the summer of 1914. The soil in which these drains were laid is 
rather close but has been underdrained sufficiently long to be much 
more open, and to drain more readily, than in its natural state. From 
early spring till the latter part of May the soil was very dry, though 
it had been wet when the crop, oats, was planted. The rainfall during 
Tune and July was as follows: Tune 10, 0.50 in.; June 13, 2.12 in.; 
June 22, 0.50 'in.; July 6, 1.25 in.; July 12, 2.06 in., and July 16, 1.0 in. 
Each of these rains was a heavy downpour lasting but a short time. 
The water reached the surface so rapidly that a very large proportion 
of it passed away as surface runoff. So small a portion of this water 
entered the soil that at no time during June or July was there any flow 



42 

in the outlet mains of these systems. If the rate of precipitation in 
these storms had been slow it is very possible that after the rains of 
July 12 or 16, the outlet main would have been flowing full. 

In the design of an underdrainage system consideration should be 
given to all the factors which tend to control the amount of surplus 
moisture to be removed by the drains, and the rate at which it moves 
through the soil to them. Where the rainfall is such as will ordinarily 
cause the saturation of the soil to a high level during a considerable 
period, the spacing of the laterals must be such as will cause a rapid 
movement of the groundwater to the drains and the runoff coefficient 
used in calculating the sizes of the tile required must be correspond- 
ingly large. 

Topography of the Watershed: The area and slopes of the water- 
shed have a very great influence upon the amount of surplus mois- 
ture to be removed by the drains. If the slopes are steep a large part 
of the rainfall passes away over the surface. If the underdrained area 
is a flat with an untiled tributary area with steeper slopes, a portion 
of both the surface and natural underground runoff from the tribu- 
tary area will be collected in and on the flat. This condition is the 
same in effect as subjecting the flat to heavier rainfall. The amount 
of such increase will depend upon the slope and proportional area of 
the tributary watershed. The effect of such additions upon the design 
of the lateral system and upon the rate of runoff has already been dis- 
cussed in the previous paragraphs. 

The size of the underdrained area, unless it be very large, will us- 
ually have but little effect upon the rate of runoff. The time neces- 
sary for water to pass through the drains themselves is very small 
compared to that required for the passage through the soil to the 
drain. If the spacing and depth of drains, the soil conditions and 
the relative elevation of the plane of saturation are the same in all 
parts of the underdrained area, the flow will reach all laterals at the 
same rate. The rate of the movement of the water through the 
drain is so rapid, compared with this movement through the soil, that 
it seems very probable that the rate of runoff will not be affected ma- 
terially by the size of the underdrained area. It must be remembered 
however, that this statement is true only when applied to areas exactly 
similar in all respects except size. 

The effect of topography should be studied, in each particular case, 
to determine its relation to an increase or decrease of the normal 
amount of surplus moisture and then the underdrainage designed to 
remove the surplus moisture at the desired rate. 

Character of Soil: The relation of the properties and characteristics 
of soils to the rate of movement of soil moisture has already been 
discussed. Since the rate of runoff is but another expression of the 
rate of the movement of the surplus moisture through the soil, these 
discussions need not be repeated here. Considering each of the three 
physical properties of soil separately, the other two being constant 



43 

in each case, the rate of runoff varies inversely as the size of the soil 
grains, directly as the closeness of their arrangement and directly as 
the porosity. 

The foregoing statements mean that a fairly fine grained soil that 
is loose will store a considerable quantity of moisture and will allow of 
its percolation at a fairly rapid rate; that a fine grained, close soil, 
such as clay or close "gummy" black soil, will allow of only a very 
slow percolation but has a large reservoir capacity, though consider- 
able time is required for the pore spaces to become filled ; that a large 
grained porous soil, as sandy loam, will hold but a small quantity of 
drainage water in storage, but allows of its percolation at a rapid rate. 
The relation between rainfall, including surface runoff collected on 
the underdrained area, the storage capacity and the rate of percola- 
tion determines the rate of underdrainage runoff. 

Evaporation and the Transpiration of Plants: The amount of water 
which would otherwise reappear as runoff, but which is removed from 
the soil by these two agencies, varies with the climate, the nature and 
the amount of the vegetation and the character of the soil. It should 
be remembered, however, that unless the soil is saturated clear to the 
surface so that evaporation may take place directly, neither of these 
agencies will affect the amount of drainage water except indirectly. 
The relation of evaporation and the use of water by vegetable life, to 
the moisture content of the soil has been discussed in previous chap- 
ters which need not be repeated here. It has been estimated that, 
in the Upper Mississippi Valley, the average maximum daily loss 
from these two causes will not exceed one-tenth of an inch. How- 
ever, both of these depend to a large extent on the plant growth. The 
water required by some plants is much more than that necessary for 
others. If the vegetable growth is dense and rank, the shading of the 
surface may be such as to reduce evaporation to a minimum. 

The amount of water removed annually from the soil by evapora- 
tion and used by plants may be large in amount, but it is always small 
in comparison to the underdrainage runoff. 

Climate and the Seasons: That the climate and the seasons have an 
important bearing upon the rate of runoff is readily seen from the dis- 
cussions already given. In Iowa in the early spring the rainfall is or- 
dinarily large and the losses through evaporation and transpiration 
are small, causing a comparatively high rate of runoff. In the sum- 
mer the rainfall may be large, but these losses are also large, result- 
ing in a lower rate of runoff. In the fall, if the amount of rainfall be 
large the rate of runoff will also be high, as the evaporation and 
transpiration losses are small. In other words, the effects of climate 
and seasons upon the rate of runoff are the combined effects of the 
resulting rainfall and the evaporation and transpiration losses. 

29. Economic Rate of Runoff: Conditions sometimes prevail such 
that to provide capacities in the drains equal to the flow of surplus 



44 

moisture resulting - from the maximum rate of rainfall, would require 
a larger investment than that upon which the average benefits from 
the drainage would pay an adequate return. These unusually severe 
storms, or series of storms, may occur at such infrequent intervals 
that it is economically desirable to suffer whatever loss may be due 
to the inadequacy of the drains during such storms than to provide 
the drainage system necessary at such times. The drainage systems 
should be designed to care for the maximum runoff for whcih it is 
economically possible to make provision. This rate of runoff, some- 
what below the actual maximum, is often referred to as the economic 
rate of runoff. 

In this connection it should be remembered that the more rapidly 
the surplus soil moisture is collected in the laterals, the higher will 
be the rate of runoff for which the capacities of the various lines 
should be calculated. Higher values for farm land and products make 
more efficient, more rapid, and more thorough underdrainage both de- 
sirable and possible, economically. The increasing of the general 
knowledge of underdrainage and its benefits are responsible for a con- 
stant tendency toward more efficient drainage. All of these things 
will demand that the mains and submains be designed to care for 
larger rates of runoff. As an underdrainage system properly con- 
structed of good materials will last for a great many years, it will be 
cheaper to provide at first for a larger rate of runoff than seems neces- 
sary at that time, than to make replacements or additions within a 
few years when the recognized conditions demand the more effi- 
cient drainage service. 

30. Runoff Data and Values: The designers of farm underdrain- 
age systems have been confronted with a paucity of reliable data as to 
the runoffs from underdrained areas. Consequently they have had to 
depend upon observation of the operation of existing systems, making 
the capacities larger or smaller than those in some other system as 
the conditions appeared to demand. This condition has led to much 
work being designed for the care of a certain rate of runoff without 
really studying the particular project, or its differences from some 
other system. It has been the purpose of the foregoing pages of this 
bulletin to explain the whole subject so that the designer might make 
a rational study of each project, or proposed system, and then design 
the underdrainage system, using all available data, from publications 
and observations, with full knowledge of the significance of all espe- 
cial conditions obtaining in the particular problem. 

The data as to the rates of runoff from farm underdrainage sys- 
tems given in Bulletin 52, Iowa Engineering Experiment Station, 
comprise practically the only published, comprehensive records for 
such systems. A few other data from older investigations, principally 
of larger county systems, are to be found, but because of their nature 
and the general advancement in drainage practice, these are now not 



45 

applicable without making many allowances for the advancement in 
drainage work. 

The data in the publication just referred to show that the rate of 
runoff now generally used is not applicable to all cases, or, that the 
tvpe of the lateral system and the completeness of the drainage have 
a marked effect upon the rate of flow to the drains. In fact, these 
data are concrete evidence of the influence of nearly all the factors 
discussed in the foregoing chapters. 

These data show that if one rate of runoff is to be used as a basis 
from which to reach decision in each particular problem, this rate 
should be slightly larger than the rate of % inch now in quite com- 
mon use ; in some cases this rate will need to be increased materially. 
The tendency is ever toward more thorough drainage, so that con- 
siderable care should be exercised to insure that the main drains of 
the system will have sufficient capacity for the completed and ex- 
tended system of the next few years. Much of the benefit of closer 
lateral spacing may be counteracted by mains of too small a 
capacity. 

The problems encountered in the design of a large county drain 
are capable of less exact study. The flow through the main lines of 
a county drain are only the combined flows from the various separate 
farm systems. These drains now usually are provided with inlets 
for admitting surface water, which add to the problem of deciding 
upon the probable capacity required. It is obviously impracticable 
and impossible for the engineer to study each individual farm and at- 
tempt to decide upon the rate of flow from the underdrainage sys- 
tem which the owner will install. Some landowners will install no 
farm drainage systems ; others will construct more or less complete 
systems, depending upon their opinions as to the needs for under- 
drainage and the value of the systems to their farms. From his 
knowledge of the general underdrainage requirements and the local 
sentiment for and against thorough underdrainage, the engineer 
should be able to reach a fairly accurate conclusion as to the capaci- 
ties to be required in the main lines of the county system. He 
should not forget, however, that there is a growing knowledge of 
underdrainage, with a resulting demand for ever increasing thorough- 
ness of drainage. It is the moral duty of every engineer to be a con- 
stant advocate of more thorough drainage in his community. There 
is now no apparent reason to expect the present tendency toward bet- 
ter drainage to reach its climax or to turn for many years. The effect 
of this growth and development should not be ignored when design- 
ing any drainage system. 

In designing large county drains provided with intakes for admit- 
ting surface water the engineer should increase his underdrainage rate 
of runoff sufficiently to care for the surface flow admitted. If this is 
not done the whole capacity of the drain will often be used for the 
surface flow and the tributary underdrains remain inoperative till this 
accumulated surface flow is removed. 



46 

VI. FLOW IN UNDERDRAINS. 

The foregoing pages have presented a discussion of the forms of 
soil moisture, their movements, their relation to the various soil prop- 
erties, crop production and drainage, the relationship of the rate of 
movement to the surplus moisture, crop requirements and soil prop- 
erties to the form of the lateral system and the resulting relationship 
of all these to the rate of runoff, or to the capacities required in the 
various lines of the system. This section will take up a discussion of 
the factors controlling the rate of flow through the drain and the 
methods of computing this. 

31. Cause of Flow and Factors Affecting Rate : The pull of grav- 
ity is the sole cause of the flow of water, except where pumps of some 
sort are used. It is gravity which causes water to run down hill or a 
ball to roll down an inclined plane. 

The rate, or velocity, of the movement in the case of either the 
water or the ball, depends upon the steepness of the slope down 
which the movement takes place and the resistances which the path 
offers. In the case of the flow through a tile drain, the velocity de- 
pends upon the slope, or grade, of the moving water and the rough- 
ness of the inside of the drain. Obviously, the quantity of flow per 
unit of time is the product of the cross-sectional area of the moving 
column and the rate of movement. 

Hydraulic Grade: In these, as in all such hydrological problems, the 
hydraulic slope or grade is the one which determines the velocity of 
the flow. The hydraulic head, when flow is due to gravity only, is 
the difference in elevation of the free water surface at the two ex- 
tremities of the portion of the flow under consideration ; the hydraulic 
grade is the quotient obtained by dividing the hydraulic head by the 
horizontal distance between the upper and lower limits of this flow. 

This hydraulic grade may or may not be parallel to the grade at 
which the tile were laid ; it will be parallel to this constructed grade 
only when the tile are flowing just full at both ends and neither end 
is submerged. However, it is customary to calculate the capacity of a 
drain when it is flowing full at a grade equal to that at which the tile 
were laid. 

Resistance to Floiu: The principal resistances to flow in a tile drain 
are the roughness of the inside of the tile, the irregularities due to 
careless laying, the irregularities due to tile of nonuniform shape and 
size and sharp or irregular changes in direction. The roughness of 
the inside of the tile is an inherent quality or property of the tile being 
used, and can be changed only by the action of the flow. The irregular- 
ities of laying cannot be entirely eliminated, but by careful workman- 
ship this resistance can be reduced very greatly. The irregularities 
due to unsymmetrical or ununiform tile can be almost wholly elimi- 
nated by proper inspection of the tile before they are laid. Any channel 
composed of unconnected short sections, as a tile drain, will have 



47 

larger resistance to flow than a channel composed of a continuous 
tube of like material. The resistance due to sharp and irregular bends 
can be greatly reduced by making all changes in direction by means 
of longer, easier and regular curves ; changes in the direction of the 
drain .cannot be avoided practically, but by using proper curves this 
resistance is reduced to a very small amount, both actually and com- 
paratively. 

All formulas for the flow in a tile drain take into account the effect 
of these resistances, usually by means of a constant which varies, with 
the roughness of the inside of the drain, or with the size of the tile, or 
both. In designing a drain it is customary to consider the resistances 
present in a drain constructed of average good materials and in an 
average good, workmanship-like manner. 

32. Formulas for Flow in Underdrains : All formulas used for 
calculating the velocity of the flow through a tile drain have been de- 
rived, partially at least, from the results of experiments. For this rea- 
son, all such formulas contain constants whose values must be chosen 
with reference to the conditions obtaining in the problem to be 
solved. (The constant mentioned above as expressing the effect of the 
resistance due to the roughness of the inside of the drain is an ex- 
ample.) Great care must be exercised in choosing the values of these 
constants in order that the values which are truly applicable be used. 

Several different formulas are used, by drainage engineers in Iowa, 
for computing the capacities of tile drains. However, as only two of 
these are in general use, only these two will be discussed here. Any 
one wishing to study other formulas can find them stated and ex- 
plained in published works on hydraulics. 

Ratter's Formula: Probably the most widely used and most reliable 
formula for calculating the flow in tile drains is a combination of the 
simple basic formula proposed by Chezy and the more complex for- 
mula proposed by Kutter and Ganguilet for determining the value of 
the constant in the Chezy Formula. This combination is usually 
spoken of simply as Kutter's formula. Some new formulas for the 
flow in circular channels have been derived and found to be more 
satisfactory than Kutter's formula for the particular cases studied. 
As yet no new formulas especially adapted to the calculation of flows 
in tile drains have been published. Kutter's formula is probably ac- 
cepted as the most accurate one available in a very great majority of 
such hydraulic problems. 

Chezy's Formula is : 

V=CVr^ 
where V=the velocity in feet per second, 

C=a constant, depending primarily, on the resistance to flow 

from different causes, 
r=the mean hydraulic radius, 
s=the hydraulic slope or grade. 



48 

The constant in this formula was determined by Kutter and 
Ganguilet to be : 

.00281 1.811 
41.6 + + 



C= 



.00281 
(41.6+ )n 



1 



where s=the hydraulic slope, 

n=the coefficient of roughness, 
r=the mean hydraulic radius. 

The mean hydraulic radius, r, is determined by dividing the area 
of the cross-section of the flow by the area of the wet perimeter per 
unit length of channel. In circular pipes flowing full, r equals one- 
fourth the internal diameter. 

The value of the "coefficient of roughness," n, varies from .010 for 
very smooth continuous pipe to .020, or even more, for poorly con- 
structed drains. The best available information and the best drainage 
practice favors the use of a value of .015 for the coefficient of rough- 
ness of average good tile drains. 

In Table XI is given summary of the results obtained in an in- 
vestigation of the value of "n" for several Iowa drains. These data 
were collected in 1909 by Messrs. Rightmire and Chappel as a thesis, 
the work being done under the direction of the Iowa Engineering Ex- 
periment Station. The data given below are taken from the complete 
report of these investigations given in the Engineering Experiment 
Station Bulletin, Vol. IV, No. IV. 

In these investigations the discharge from the drain being investi- 
gated was measured by means of a weir. The hydraulic slope and 
depth of flow were determined for five consecutive stations of about 
equal length. With these data the value of "n" for each station was 
computed, and the average of these taken as the value for the drain. 

TABLE XI. 
THE COEFFICIENT OF ROUGHNESS IN AVERAGE IOWA DRAINS. 

Size Material "n" Character of Grade Depth of Flow 

2'x32" Cement .01504 Irregular 23y 2 " to 24 1-3" 

2'x24" Cement .01638 .01% to .09%, irreg. V 2 " to 2%" 

2'x20" Cement .01146 Regular 7%"to9%" 

2'xl8" Clay, hard burned .01172 .3% to .6%, regular 4" to 7%'' 

2'xl4" Clay, hard burned .01525 1% to 2%, irreg. 1%" to 3" 

l'xl2" Clay, hard burned .01683 .15% to 1.1%, irreg. 1 X A" to 2%'< 
1'xlO" Clay .0164 1.2% to 1.7%, fairly 

irregular % to 1' 

These data are too few to be conclusive, but they seem to bear out 
the use of the value of .015 for "n" with well laid Iowa drains. It is 



49 

quite certain that this value of "n" is too large for some very care- 
fully constructed drains, but in such cases the error is not large and 
is on the side of safety. As far as is shown by the above data, the 
or 1 ' variation of "n" is with the regularity of the grade and possibly 
n the length of the individual tile, as the number of joints is in- 
... ased with shorter tile. 

Use of Mutter's Formula: In actual use of this formula the value 
of the constant "C" is not computed, but is taken from either a dia- 
gram or from tables. Having the value of "C" the velocity is easily 
computed, or taken from a table, and from it the discharge determined. 
Tables and Diagrams giving the values of "C" for different slopes 
and depths of flow are given in many texts and handbooks, so are 
not repeated here. Trautwine's "Civil Engineer's Pocket Book" gives 
very complete data of this kind. 

Another method of simplifying the use of this formula is by means 
of diagrams, or curves, which show the discharges for various sizes of 
drains at various slopes. These diagrams can be prepared for any 
shaped channel with any depth of flow at any slope, and for any de- 
gree of roughness as indicated by the coefficient "n." Such a dia- 
gram for tile drains from 4 inches to 48 inches in diameter flowing 
full at hydraulic grades of from 0.02 feet to 5.0 feet per 100 feet, when 
"in" equals .015, is given as Fig. 8. At the right of the diagram has 
been added a scale showing the number of acres drained, at different 
rates of runoff, for different discharges in cubic feet per second. This 
diagram is as accurate as is possible considering the probable errors 
of plotting and reading the curves. Any engineer desiring to use 
such a diagram in his work should prepare one to a larger scale on 
mounted paper which is not subject to great change under different 
temperature and humidity conditions. 

In preparing these diagrams it is preferable to plot them to a 
logarithmic scale, as this gives a straight line for the discharge for 
each size of tile. In this way the diagram may be prepared by plot- 
ting the discharge for each size of tile at three grades and then con- 
necting these points with straight lines. The scales at the right of the 
diagram are prepared simply by platting the number of acres 
drained at different rates of runoff opposite the equivalent discharge 
in cubic feet per second. 

This use of the diagram may be explained by two examples : 

1. What is the discharge from a 15 inch drain at a grade of 0.20% 
or 0.20 feet per 100 feet? Follow up the vertical line from the notation 
0.20 at the bottom of the diagram till this line intersects the dis- 
charge line for 15 inch tile and then across horizontally to the scale 
at the left, where the result is found to be approximately 2.28 cubic 
feet per second. 

2. What size of tile, laid at a .015% grade, will be required to 
drain 500 acres at a j^-inch runoff? Follow up the vertical line repre- 
senting a grade of .015% till this intersects the horizontal line from 



Discharge in Cubic Feet per Second 



$ § SS § 




1 ' ' ' | ■ ■ ' ' | 'I I I | I I I I | I I I I | I I I I | II I I | — I | I | I | I | I | I I I I | | I I I | I I I l | II M | I I II 



,,*,* ,, 5 § g § §■ g § 8 8 § 8 I 8 § 8 

' ' ' ' ' ' ' ' l I ! I | i i i i | I I I I | ii I I 1 1 1 i i| — | | I j I | i | i | r t i i | i i i i | 1 1 i i [ 1 1 n [ i i M 

2 ° ° ooooooo o 855 



c 
> 



sg 

H fD 



50 

500 acres at a ^-inch runoff on the scale at the right. This intersec- 
tion shows that a 24 inch tile is slightly too small and a 26 inch tile 
too large. The engineer must use his judgment in deciding between 
these two sizes, though in a case of this kind, it is often better to 
have drains too large rather than too small. In designing a drainage 
system all drains should be designed to have the required capacity 
when the hydraulic grade is parallel to the constructed grade, or 
when flowing full but under no head other than that afforded by the 
slope of the drain. If this is done, the drain will have extra capacity 
for the flow from unusual storms and spring freshets when the water 
will be backed up at the upper end of the drain causing a steeper 
hydraulic slope. Another reason for not designing the drain to work 
under extra head is that the pressure will often cause water to pass 
cut through cracked tile and wash or soften the earth so as to allow 
these cracked tile to spread and collapse. 

Poncelefs Formula and Elliott's Modifications: The other formula, 
or rather a set of formulas, in general use among Iowa drainage engi- 
neers is termed, very often incorrectly, Elliott's formula. This set of 
four formulas proposed by Mr. C. G. Elliott, formerly chief of Drain- 
age Investigations, U. S. Department of Agriculture, consists of Pon- 
celet's formula and three modifications suggested by Mr. Elliott. 

Poncelet's Formula: 



/d h 

v = 48 V an d Q — av 

1 + 54 d 

where v=the velocity in feet per second, 

d = the inside diameter of the tile in feet, 

h = the head, or difference in elevation between the outlet and 

the upper end, in feet, 
l=length of the drain considered, in feet, 
a = the area of the cross-section of the flow in square feet, 
Q=the flow in cubic feet per second. 

In order to give consideration to some other factors Mr. Elliott 
suggested three modifications of Poncelet's formula, each of the four 
to be used under certain conditions. He first introduced a factor "k." 
or fractional part of "k." This factor represents the depth of earth 
over the upper end of the drain. If the soil is loose and becomes sat- 
urated the water therein was taken as creating an additional "head" 
which would increase the flow through the drain. In loose soil he 
recommended that "dh" be increased by J / 2 "k." 

Another factor which he considered as increasing the capacity of 
the drain is the increase in velocity which may be caused by the num- 
ber and grades of the submains. In order to take this into account 
further modification is suggested by replacing "h" by "the quantity." 



51 



h + - 

n 
in which h = the head, in Poncelet's formula, 

b == the sum of the amounts of the excess head in the sub- 
mains, 

n = the number of submains. 

The amount of excess head in each submain is the difference be- 
tween the head on it and that for a like length of the main. 

Mr. Elliott recommends the uses of these four formulas as follows : 

Poncelet's formula: 



v = 48 V 



/ dh 



for use in small systems in close soil and, 



l-(-54d as a general rule, for outlet mains. 
Elliott's modifications 



v = 48 V 



/dh + y 2 k 
l+54d 



for use in systems where soil is open. 



/ b 

/d(h+-) 
48 V n 



for use in large systems in close soil. 



l + 54d 



and v 



/ b 

/d(h+-)+/ 2 k 
48 V n for use in large systems in open soil. 



l+54d 

Trawtwine's "Engineer's Pocket Book" recommends that for 
pipes larger than 12 inches in diameter the following numbers be 
used instead of the constant 48: 

For 18 inch pipe, coefficient, 53 



24 ' 


ia t 


57 


30 ' 


( n i 


60 


36 ' 


( (t c 


62 


42 ' 


I 11 I 


64 


48 ' 


( (C t 


66 


60 ' 


its I 


68 



Use of "Elliott's" Formula: The use of either of these formulas may 
be simplified by the preparation of tables or diagrams showing the 



52 

discharge in cubic feet per second, or the acres served at a given rate 
of runoff, by various sizes of drains at different grades. A separate 
discharge table must be prepared for each formula. A separate "acres- 
drained" table must be prepared for each formula and each rate of 
runoff used. 

The general discussions as to the use of Kutter's formula apply 
equally when either of these formulas is used. 

33. Advantages of Each of the Two Formulas: Hydrologists 
quite generally consider Kutter's formula more accurate than Ponce- 
let's for calculating the flow through pipes. Authorities on sewer de- 
sign generally consider that the flow through a sewer is given more 
accurately by Kutter's formula than by Poncelet's ; practically all texts 
on sewer design recommend the use of either Kutter's formula, with a 
value of .015 for "n" for pipe sewers, or of some form of exponential 
formula which has been especially adapted to sewer design. 

The accuracy of Elliott's modifications of Poncelet's formula is 
often questioned on the ground that they are based on unsound prem- 
ises. It is probably true that under certain extreme conditions, the 
soil above the drain may become so saturated that the water in this 
soil would tend to increase the head on the drain. This condition 
would not obtain, however, if the drains had the capacity to remove 
the water as rapidly as it passed through the soil to them. No water 
which is flowing has any actual head upon it unless the drain is sur- 
charged ; up to the capacity of the drain, all static head is converted 
into velocity head or used in overcoming frictional resistance. In other 
words, it would seem that the premise on which Elliott's first modi- 
fication is based could be true only when the water is moving through 
the soil to the drain more rapidly than it can flow away through the 
drain. 

Elliott's second modification is made to care for an increase in 
velocity due to the fact that the submains have a steeper grade than 
the main. This case is very similar to that just discussed except that, 
when the velocity in the submain is greater than through the main, 
this excess in velocity is converted into static head when the flow 
passes into the main. The extra head so created would, however, be 
small, and it is difficult to see how the velocity in the main will be 
increased materially by that of the submains so long as the main has 
sufficient capacity to care for all the flow entering it without backing 
water up in the submains. 

The use of tables and diagrams makes these two formulas (Kut- 
ter's and Poncelet's) equally easy to use. Familiarity with the tables 
or diagrams of either enables the engineer to use either speedily and 
accurately. It is obvious that if the capacities are to be determined 
by solving the complete formula in each calculation, either of Elliott's 
formulas can be used with much less tedious labor than Kutter's. 

For small tile Elliott's basic formula (Poncelet's) gives larger 
capacities than Kutter's ; the difference in the results by the two 



53 

formulas decreases as the size of tile increases so that for 20 inch or 
24 inch tile they give very approximately the same result; for the 
larger sizes there is little difference, comparatively, though Kutter's 
formula gives the slightly larger capacities. The fact that Elliott's 
formulas allow the use of smaller sizes of tile, within the range of 
sizes common to farm tile systems, than Kutter's is an argument 
against them as the sizes used in such systems are too small much 
more often than they are too large. 

VII. RESULTS OF UNDERDRAINAGE. 

The results, or effects, of underdrainage may be divided into the 
three classes ; financial, physical and general. However, any such 
classification must be quite elastic, as the financial and general results 
are due either directly or indirectly, to the physical changes brought 
about by the construction and operation of the underdrainage system. 
On the other hand, the resulting financial gain or loss is usually the 
most apparent result of the prosecution of the drainage work. 

34. Financial Results : The financial returns from any drainage 
improvement work will usually be taken as a measure of the success 
of the work. Probably the only exception to this is found in the com- 
paratively rare cases where the drainage is resorted to as a means of 
protecting the health of the community and for no other purposed 
Even in such cases, the probable value of the reclaimed land is usually 
given serious consideration before the work is started. 

When a drainage improvement is proposed the owner of each 
parcel of land to be affected by the proposed drain will ask himself 
the question, "How and to what extent will this drain benefit me?" 
The direct results of the construction of the drainage system will be 
changes in the physical condition of the lands drained. Certain of 
these physical changes will afford the landowner a financial gain. 

The removal of the surplus water from a piece of wet lartd will 
change this tract from an unproductive to a productive area. By rea- 
son of this change the actual sale value of this land will be increased 
in proportion to its increase in productiveness. 

This increased productiveness will make possible a direct financial 
gain in the greater value of the crops raised. Proper drainage will 
make the cultivation less difficult, thus making the cost of producing 
the large crop less than that of raising the small crop. 

Another, though smaller, financial gain from proper drainage re- 
sults from the lessened cost of transporting the farm produce to a 
market or shipping point. It is a well proven fact that good and 
economical highway construction must be preceded by drainage. The 
removal of the surface water by open ditches is no more important 
in this respect than the removal of the surplus ground water so as to 
provide a firm foundation for the roadway. The cost of hauling the 
farm crops over a good highway is much less than that of hauling 



54 

this same crop over a poorly constructed and poorly maintained road. 
If improper drainage works are constructed, the financial results 
will be smaller or may even constitute a loss. If the conditions are 
not improved by the construction of the drain or drains, the cost of 
the work and the loss of the benefits which would have resulted had 
the work been properly designed and constructed will constitute no 
small loss. 

35. Physical Results: The direct effects of underdrainage are 
the physical changes which are due to the operation of the drains. 
The most important of these physical changes are those which take 
place within the soil, the character and mechanical composition of an 
ever-wet or water logged soil being completely changed by under- 
drainage. It is significant that all the physical changes resulting from 
underdrainage are classed as benefits by agriculturists and that many 
of these are due wholly or in part to actions which take place when 
the surplus soil moisture is removed. 

The following short discussion of the effects of underdrainage is 
but a recapitulation and application of the discussions given in the 
earlier sections of this bulletin. 

1. Improves the Physical Character or Mechanical Composition of 
the Soil: Underdrainage aids greatly in the formation of soil granules. 
The granular structure is particularly desirable in fine grained soils, 
as they then have the desirable properties of coarser grained soil in 
permitting of the rapid passage of moisture and air. One of the im- 
portant factors in soil granulation is alternate wetting and drying. 

When a soil is saturated the soil grains are held apart, and partially 
floated, by the water. In addition to this, the water acts as a lubri- 
cant so that the soil will not support the weights necessary in culti- 
vation. When the soil is saturated continually the soil granules are 
broken down and the smaller grains move into the spaces between 
the larger ones. In this condition the soil becomes almost a compact 
mass and is termed puddled. Underdrainage prevents puddling. 

These actions and conditions are often expressed by saying that 
underdrainage makes the soil more loose, more open or more mellow, 
and that standing water, or saturation, packs it. These are but dif- 
ferent methods of expressing the conditions discussed above. 

2. Improves the Aeration: Many agriculturists hold that the physi- 
cal changes which cause a more rapid and greater passage of air 
through the soil is the largest single benefit of underdrainage in that 
thorough aeration is one of the most important factors in crop pro- 
duction, in so far as this depends upon the condition of the soil. Un- 
derdrainage improves the aeration in two ways ; it removes the sur- 
plus, or gravitational moisture, thus leaving the soil pores open for the 
passage of air; and promotes soil granulation, thus providing larger 
channels for the circulation of air. 



55 

3. Increases the Supply of Available Plant Food and Moisture: This 
is probably the most important effect of underdrainage upon the soil, 
but it was not taken up till now because of its dependence upon (1) 
and (2). This effect is due to several actions, all resulting- from the 
drainage, the most important of which will be mentioned. 

It has been explained that plant roots take up food that is in solu- 
tion in soil moisture ; that they can take up this moisture only when 
it occurs in the soil as capillary moisture ; and that the capillary mois- 
ture is present only after the surplus, or gravitational, moisture has 
been removed. This chain of facts makes it at once apparent how 
underdrainage may increase the available supply of plant food. Un- 
derdrainage lowers the plane of saturation, thus making available the 
food supply of a larger volume of soil. 

By causing the formation of granules, the underdrainage increases 
the amount of capillary moisture and thus increases the food supply. 

Underdrainage improves aeration which in turn causes the forma- 
tion of additional plant food in the soil. 

Underdrainage promotes the growth of the desirable forms of soil 
organisms and retards the growth of the undesirable forms. If the 
soil is saturated, the products of these organisms are held in the soil 
causing the destruction of all organisms. These desirable organisms 
increase the amount of plant food in the soil. 

Underdrainage increases the supply of plant food through chemical 
changes which are dependent upon this moisture condition. 

It has already been explained that underdrainage causes certain 
changes in the physical character of the soil and thus increases the 
maximum content of available or capillary moisture. This explains 
why crops in a thoroughly drained field withstand drouth better than 
those in a similar undrained field. Underdrainage increases the sup- 
ply of available plant moisture by removing the injurious surplus 
gravitational moisture from a wet soil and by causing physical changes 
in the soil that result in the storage of an increased supply of avail- 
able moisture in the soil after this surplus has been removed. 

The benefits of underdrainage in increasing the supply of plant 
food and moisture are well illustrated by Fig. 9. The measurements 
from which this diagram was prepared were taken at the foot of a 
slope where possibly seep water was present. It will be noticed that 
the stalks of corn grew much stronger directly over the lines of tile 
where the drainage was most thorough. 

4. Raises the Average Temperature: The specific heat of water is 
much higher than that of soil. The greater the proportion of water a 
soil contains the more heat is required to raise the temperature of the 
sample a given amount. The continual evaporation from the surface 
of a wet soil reduces the temperature or retards the increase in tem- 
perature. The heat applied to a given soil area is fairly constant, so 
that if this heat must be used up in evaporating water the temperature 
of the soil body is not raised. 



56 



Drainage removes the excess water from the soil, reducing- the heat 
required for evaporation and causing the soil body to warm up more 
readily. As a result of this, a drained soil warms up much earlier in 
the spring, and so lengthens the growing season. This enables the 
farmer to start his spring work earlier, which is especially valuable 




Fig. 9. Diagram Showing the Effects of Drainage Upon the Height of Corn at 
Hanford, Cerro Gordo County, Iowa, October, 1910. 

in a so-called "backward" spring. The growth of the crop upon 
drained land is greatly benefited by the high temperature that pre- 
vails here in the spring and fall. Investigators have found that at a 
depth of seven or eight inches a drained soil is from 12° to 15° warmer 
than an undrained soil of the same nature and in the same climate. 

5. Reduces Heaving: It is often noted that posts have been raised 
out of the ground during the winter. This heaving is due to the freez- 
ing of a wet soil. When water freezes it expands one-eleventh of its 
volume and in a saturated soil this expansion must be upward, the 
amount of the heaving depending upon the amount of water in the 
soil and the depth to which it is frozen. This same action tends to 
raise the roots of certain crops out of the ground, as the soil settles 
back after thawing, more rapidly than the plant root. This heaving 
also breaks many of the small roots. 

6. Reduces Erosion: In an undrained area all of the rainfall must 
either be absorbed by the soil or pass away over the surface. In a 
continued rainy season the soil soon becomes saturated after which 
all the rainfall must flow away over the surface. The particles of the 
saturated soil are easily displaced and carried away by the water. 

In an underdrained area the soil has a greater water capacity and 
allows of a continual removal of the surplus water by the drains. 
This greatly reduces the amount of water which must pass away 
over the surface and thus reduces erosion. Underdrainage of slopes 
will often prove a profitable investment if installed for no purpose 
other than reducing or preventing erosion. 

36. General Results: Those results of the physical changes 
brought about by drainage, to which cannot be readily assigned a 



57 



monetary value, are termed general benefits. In the well drained 
section the better highways allow of more, easier, and more rapid 
travel. The benefits of this in a social way are no small matter. The 
rural mail service demands better highways than are usually possible 
without both open channels and underdrainage in many sections. The 
consolidated school depends upon the possibility of transporting the 
children to and from school, and this in turn depends upon good roads. 
The appearances and generally better financial condition in the well 
drained territory add to the value of the farm. The sanitary condition 
in wet or swamp areas are bettered materially by both open ditches 
and underdrainage. 



LIBRARY OF CONGRESS 



000 936 716 



The College 



The Iowa State College of Agriculture and Mechanic Arts con- 
ducts its work along five major lines : 

Agriculture 

Engineering 

Home Economics 

Industrial Science 

Veterinary Medicine 

The Graduate Division conducts advanced research and in- 
struction in all these five lines. 

Four, five and six-year collegiate courses are offered in differ- 
ent divisions of the College. Non-collegiate courses are offered 
in agriculture, engineering and home economics. Summer Ses- 
sions include graduate, collegiate and non-collegiate work. Short 
courses are offered in the winter. 

Extension courses are conducted at various points throughout 
the state. 

Research work is conducted in the Agricultural and Engi- 
neering Experiment Stations and in the Veterinary Research 
Laboratory. 

Special announcements of the different branches of the work 
are supplied, free of charge, on application. The general cata- 
logue will be sent on request. 

Address The Registrar, 

Ames, Iowa. 



